Effects of chemical substitution on the structural and optical properties of -Ag22xNixWO4 (0 x 0.08) solid solutions

In this work, we investigated the effects of chemical substitution on the structural, electronic, and optical properties of a-Ag2 2xNixWO4 (0 r x r 0.08) solid solutions prepared by a facile microwaveassisted hydrothermal method. The results showed that the increase of Ni concentration in a-Ag2WO4 microcrystals as a host matrix caused a morphological transformation and a shift of the electronic and optical properties. Based on first principles calculations and using Wulff’s construction, particle shapes and their transformations in a-Ag2WO4 and a-Ag2 2xNixWO4 can be affected by controlling the ratios of surface energy values between the different facets. In addition, theoretical calculations revealed that Ni substitution in a-Ag2WO4 is more favorable in the Ag2 and Ag4 positions, in which the local coordination of Ag atoms corresponds to clusters with coordination numbers of seven and four, respectively. This behavior could be related to the degree of medium-range structural disorder in a-Ag2 2xNixWO4 crystals. The experimental results were correlated with theoretical simulations to achieve a deeper understanding of the relationship between morphology and properties. These results provide the basis for a rational design for the compositional modulation of structural and optical properties.


Introduction
Silver tungstate (Ag 2 WO 4 ) based materials have been the subject of extensive research because of their excellent applications in different branches of science and technology, such as sensors, catalysts, bactericides, and photoluminescence. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18] Their excellent properties can be attributed to the flexibility of W and Ag in adopting the [WO 6 ] and [AgO n ] (n = 2, 4, 6, and 7) coordination modes, which can be linked further by other polyhedra to form three-dimensional structures. 2,9,19,20 Notably, bulk Ag 2 WO 4 can be found in three main phases: a-orthorhombic (with space group Pn2n), b-hexagonal (with space group P63/m), and g-cubic (with space group Fd3m); 21 under ambient conditions, a-Ag 2 WO 4 is the most stable polymorph. 8,21,22 In this context, well-defined a-Ag 2 WO 4 crystals with various morphologies and sizes have been successfully synthesized using a variety of methods. 2,21,23 However, less attention has been devoted to a-Ag 2 WO 4 crystals doped with different types of impurities. 24 Generally, successful doping requires substituent and host ions with comparable ionic radii and charges, allowing only a small disturbance in the crystal framework. This has been achieved in a-Ag 2 WO 4 , whose crystal structure is extremely flexible. The main purpose is to introduce additional bands and energy levels inside the intrinsic band gap, and thereby significantly change the electronic properties. Therefore, the rational control of the synthesis of solid solutions based on a-Ag 2 WO 4 crystals has become a challenge, as it represents an important step in the development of new technologies based on a-Ag 2 WO 4 materials.
Herein, for the first time, we report the facile synthesis of a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) microcrystal solid solutions by means of the microwave-assisted hydrothermal (MAH) method. We investigated the effect of chemical substitution on the structure, electronic, and optical properties, and a correlation between morphology and photoluminescence (PL) was found. Field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) X-ray diffraction (XRD) with Rietveld refinements, micro-Raman (MR) spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, ultraviolet-visible (UV-vis) spectroscopy, and photoluminescence (PL) measurements were employed to characterize the samples. First-principles totalenergy calculations were performed within the periodic density functional theory (DFT) framework using the Vienna ab initio simulation package (VASP). The paper is organized as follows: Sections 2 and 3 describe the experimental and theoretical details, respectively. Section 4 contains the results together with a discussion concerning the structure, morphology, and photoluminescence properties. Comprehensive and combined experimental and theoretical results are reported to understand the relationship between photoluminescence properties and orderdisorder effects at short and medium ranges.

Experimental details
Synthesis of a-Ag 2À2x Ni x WO 4 microcrystals The starting precursors used in the synthesis of a-Ag 2À2x Ni x WO 4 microcrystals were sodium tungstate dihydrate (Na 2 WO 4 Á2H 2 O) (99.5%, Sigma-Aldrich), silver nitrate (AgNO 3 ) (99.8%, Sigma-Aldrich), and nickel(II) acetate tetrahydrate (Ni(OCOCH 3 ) 2 Á4H 2 O) (99.0%, Sigma-Aldrich). The a-Ag 2 WO 4 microcrystals were synthesized at 140 1C for 1 h by using the MAH method. Briefly, 1 mmol of Na 2 WO 4 Á2H 2 O and 2 mmol of AgNO 3 were separately dissolved in plastic tubes with 50 mL of deionized water. Under vigorous magnetic stirring, the second solution was quickly added into the first solution. The resulting suspension (100 mL) was stirred for 10 min at room temperature, and subsequently transferred to a Teflon autoclave, which was placed in an adapted MAH system (2.45 GHz, maximum power of 1050 W). Finally, the resulting powder was washed with deionized water several times, and the collected precipitates were dried at 60 1C for 12 h.
The a-Ag 2À2x Ni x WO 4 (x = 0.01, 0.02, 0.04 and 0.08) solid solutions were synthesized using 1 mmol of WO Characterization of a-Ag 2À2x Ni x WO 4 microcrystals The obtained powders were structurally characterized by XRD using a Rigaku-DMax/2500PC with Cu Ka radiation (l = 1.5406 Å) in the 2y range from 10 to 701 with 0.21 min À1 and a step of 0.02. Phase analysis by the Rietveld method 25 was conducted using the General Structure Analysis System (GSAS) software with the EXPGUI graphical interface. 26 The Rietveld routine adopted a 2y range from 101 to 1101 with a scanning speed of 0.011 min À1 . The theoretical diffraction pattern was taken from the Inorganic Crystal Structure Database (ICSD) no. 4165, 27 which is based on the orthorhombic structure with the space group Pn2n. The refined parameters in the a-Ag 2À2x Ni x WO 4 samples were the scale factor, background fitting with the Chebyschev polynomial of the first kind, shift lattice constants, profile halfwidth parameters, isotropic thermal parameters, lattice parameters, strain anisotropy factor, preferential orientation, and atomic functional positions. The peak profile function was modeled using a convolution of the Thompson-Cox-Hastings pseudo-Voigt (pV-TCH) function with the asymmetry function described by Finger. 28 FT-IR spectra were recorded in the range of 250-1000 cm À1 , using KBr pellets as a reference in a Bomen-Michelson spectrophotometer in transmittance mode (model MB-102). MR spectroscopy was conducted on a Horiba Jobin-Yvon (Japan) spectrometer coupled to a charge-coupled device detector and an argon-ion laser (Melles Griot, United States) operating at 514.5 nm with maximum power of 200 mW. The spectra were measured in the range of 25-1000 cm À1 . The morphologies were analyzed using a FEI (model Inspect F50) FE-SEM instrument operated at 5 kV. Elemental composition analysis was performed by means of EDX spectroscopy using a FEI microscope model Tecnai G2 F20. TEM analysis was performed on a CM200-Philips system and a JEOL JEM 2100F TEM/scanning TEM microscope operated at 200 kV. In addition, the optical properties of the a-Ag 2À2x Ni x WO 4 crystals at room temperature were analyzed by UV-vis and PL spectroscopies. UV-vis spectra were obtained using a Cary 5G spectrophotometer (Varian, USA) in diffuse reflection mode. PL spectra were measured using a Monospec 27 monochromator (Thermal Jarrel Ash, United States) coupled to a R446 photomultiplier (Hamamatsu Photonics, Japan). A krypton ion laser (Coherent Innova 200 K, United States) (l = 350 nm) was used as the excitation source. The incident laser beam power on the sample was maintained at 15 mW. All experimental measurements were performed at room temperature.

Theoretical details
First-principles total-energy calculations were performed within the periodic DFT framework using the VASP code. [29][30][31] The Kohn-Sham equations were solved using the generalized gradient approximation in the Perdew-Burke-Ernzerhof (PBE) formulation for the electron exchange and correlation contribution to the total energy. 32,33 Owing to the well-known limitations of standard DFT in describing the electronic structure of ''strongly correlated'' compounds, a correction to the PBE wave functions was made (PBE+U), according to the formula of Dudarev et al., 34 by including a repulsive on-site Coulomb interaction with the value of U = 6 eV for the Ag element, as in a previous study. 1 The conjugated gradient energy minimization method was used to obtain relaxed systems, which was accomplished by requiring the forces experienced by each atom to be smaller than 0.01 eV Å À1 . The electron-ion interaction was described by the projectoraugmented-wave pseudopotentials. 35,36 The plane-wave expansion was truncated at the cut-off energy of 460 eV, and the Brillouin zones were sampled using a 4 Â 4 Â 4 special k-point mesh for the calculation of the equilibrium geometries and electronic properties of the a-Ag 2À2x Ni x WO 4 structures. The substitution of one Ni atom in Ag positions (Ag1, Ag2, Ag3, Ag4, Ag5, and Ag6) was explored for the four types of possible coordination modes. To evaluate the stability of substituted structures, we have directly compared the total energies of the Ni-substituted Ag 2 WO 4 .
The crystallite shapes could be determined by using the Wulff theorem and construction. [37][38][39] The procedure to obtain the complete set of morphologies, based on the variation of the surface energies, has been previously published by Andrés et al. 17,40

Results and discussion
Structural order at long range One of the first studies on the structural determination and refinement of Ag 2 WO 4 was performed by Skarstad and Geller in 1975. 27 Today, the a-Ag 2 WO 4 structure is well known, 27 and is based on a combination of experimental data and first principles calculations. 2,9,10 19 have recently confirmed this description. Fig. 1 shows the XRD patterns of a-Ag 2À2x Ni x WO 4 microcrystals synthesized at 140 1C for 1 h by the MAH method.
The analysis of the results showed that these crystals are monophasic and can be perfectly indexed to the orthorhombic structure with the space group Pn2n, and in good agreement with the ICSD no. 4165. 27 The sharp and well-defined diffraction peaks indicated a high degree of structural order and crystallinity at long-range, proving the effectiveness of the MAH method employed in this work. Fig. S1 (ESI †) illustrates the structural refinements of a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) microcrystals synthesized at 140 1C for 1 h by the MAH method, while Table S1 (ESI †) lists the small deviations of the statistical parameters (R wp , R Bragg , R p , and w 2 ), revealing the good quality of the structural refinements and numerical results. Analysis of the results shows that a solid solution is formed without the presence of secondary phases. The Rietveld refinement method also was used to calculate the lattice parameters, unit cell volumes and atomic coordinates of a-Ag 2À2x Ni x WO 4 (with x = 0 and 0.08) microcrystals, thus analyzing the effects induced by the presence of Ni atoms. The values of the lattice parameters and unit cell volume are presented in Table 1. The substitution of Ag by Ni atoms provokes small changes in the lattice parameters and, consequently, in the unit cell volume. Moreover, variations in the atomic coordinates could be observed for the oxygen atoms, indicating the existence of possible structural distortions on the [WO 6 ] and [AgO n ] (n = 2, 4, 6, and 7) clusters (Tables S2 and  S3, ESI †) according to the replacement of [AgO n ] Â by [NiO n ] complex clusters in the lattice.
The lattice parameters and atomic positions obtained from the Rietveld refinements were used as input data in the Visualization for Electronic and Structural Analysis program, version 3.2.1 for Windows, 41 to model the orthorhombic Ag 2À2x Ni x WO 4 structure (Fig. 2). Occupancy sites of Ni atoms were investigated for a-Ag 2À2x Ni x WO 4 (x = 0.08) solid solution employing Rietveld refinements, as listed in Fig. 2.
It is important to point out that Ag1 and Ag2 atoms form distorted pentagonal bipyramid [AgO 7 ] clusters. Ag3 atoms are located at the interior of these distorted octahedral [AgO 6 ] clusters; Ag4 and Ag5 atoms are coordinated by four O atoms to form distorted tetrahedral [AgO 4 ] clusters; and Ag6 atoms are bonded to two O atoms, forming angular [AgO 2 ] clusters. 1,2,11 It was possible to estimate the distribution of Ni atoms at a concentration of 8 mol% in the atomic positions of Ag through Rietveld refinements. A distribution of Ni atoms at the available sites of each silver atom (Ag1 (4c), Ag2 (4c), Ag3 (2a), Ag4 (2a),    . 2). 1 mol%, 2 mol% and 4 mol% Ni concentrations are too low to correctly estimate the substitution process of these ions within the crystal lattice. First principles calculations were used to confirm the experimental results, and to evaluate the favorable occupation sites of Ni atoms. The substitution of a Ni atom at six different Ag positions (Ag1, Ag2, Ag3, Ag4, Ag5, and Ag6) was calculated with a concomitant Ag vacancy to compensate the positive charge balance. The theoretical results indicated that 6.25% Ni could be distributed in both the Ag2 and Ag4 positions, confirming them as the most favorable substitution sites, in good agreement with the Rietveld results. The Ni substitution in the Ag2 position was 0.3 eV more stable than that in the Ag4 position; the Ni substitutions in the remaining Ag sites appeared more difficult, owing to a difference in energy of 1.3 eV, compared with the Ag2 position. The a-Ag 2À2x Ni x WO 4 geometry showing the Ni addition and vacancy positions is depicted in Fig. 3. When an Ag2 atom is replaced by a Ni atom, the coordination changes from seven to five, with four similar distances in the range 1.89-1.99 Å and a larger distance of 2.828 Å; in this case, a vacancy of one Ag2 atom is the most stable arrangement, as shown in Fig. 3(A). However, in the case of a Ni atom replacing an Ag4 atom, the 4-fold coordination is maintained with two sets of distances of 1.900 Å and 1.949 Å; in this case a vacancy of another Ag4 takes place, as shown in Fig. 3(B). It is worth noting that after the corresponding tests, the most stable placement for the vacancy is the same type of site as the one occupied by Ni. In the two cases, Ni substitution favors the formation of more symmetric and compact polyhedra with shorter Ni-O distances compared to Ag-O distances in their respective sites.

Structural order at short range
Raman and FT-IR spectroscopies are crucial to explore the structural short-range organization. Fig. 4(A) shows the MR spectra of the a-Ag 2À2x Ni x WO 4 microcrystals (0 r x r 0.08). In these spectra, the presence of 17 Ramanactive vibrational modes at room temperature was observed, in good agreement with other studies. 10,11,42 In particular, the Raman active mode detected at B60 cm À1 (Fig. 4(A)) in the spectrum of the a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) microcrystals could be assigned to the external phonons for the motion of Ag + ions. The most intense peak centered at 883 cm À1 was associated with the vibrations of the symmetrical stretch of the ('O'W-O-) bond in the [WO 6 ] cluster. 10,11,42 Fig. 4(B) shows the FT-IR spectra of a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) crystals. While the FT-IR spectra of the a-Ag 2 WO 4 microcrystals showed only four active modes, a-Ag 2À2x Ni x WO 4 (0.01 r x r 0.08) solid solutions exhibited 13 active modes. Of these 13 modes, 11 are related to Ag 2 WO 4 and 2 modes correspond to the Ni-O bond. These results were in agreement with the findings reported in the literature. [43][44][45][46] 6 ] clusters, while the vibrational mode at 296 cm À1 corresponded to the twisting motion of [WO 6 ]. 11 The band at 635 cm À1 indicates bridging oxygen atoms in the W 2 O 2 asymmetric stretching mode. 45 Active modes at 747 cm À1 and 925 cm À1 were assigned to the vibrations of the O-W-O bonds, while the mode at 681 cm À1 corresponds to W-O-W bonds. 44,47 The absorption bands at 353 cm À1 and 442 cm À1 detected in the infrared spectra of solid solutions were due to the vibrations of the Ni-O bond, [44][45][46] indicating the replacement of Ag atoms by Ni atoms.
The presence of defined Raman and FT-IR active modes in the spectra revealed that all samples were structurally ordered in the short range. Notably, more defined spectra were observed for the Ag 2À2x Ni x WO 4 (0.01 r x r 0.08) solid solutions (Fig. 4), likely due to a decreased concentration of structural defects upon insertion of Ni atoms in the Ag 2 WO 4 lattice. However, independent of the Ni concentration, these samples are not free of structural defects, as demonstrated by photoluminescence studies. In addition, the spectra of Ni-substituted crystals seem to be more defined that could be attributed to intensity changes.

Morphological aspects
An efficient control of the size, shape, composition, structure, and surface chemistry of materials enables the modification of the physical and chemical properties. 17,[48][49][50][51] Fig . 5 shows FE-SEM images of a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) microcrystals. All samples have an agglomerative nature with a  View Article Online polydisperse size distribution and shape (Fig. S2 in ESI †). The presence of agglomerated particles, having surface defects, may arise as a result of Ostwald ripening. 52 a-Ag 2 WO 4 microcrystals have a hexagonal rod-like elongated shape, as verified in Fig. 5(A-C). According to Roca et al., 17 a-Ag 2 WO 4 crystals have preferential growth along the [010] direction. Substitution of 1 mol% of Ag by Ni atoms resulted in microcrystals with hexagonal faces agglomerated more readily, forming rod-shaped superstructures (Fig. 5(D and E)). Previous studies have shown the development of metallic Ag nanoparticles on the surface of a-Ag 2 WO 4 crystals driven by a beam of accelerated electrons from an electron microscope under high vacuum. 1,[9][10][11][12] For the sample with x = 0.01, the growth of irregular metallic Ag nanoparticles on the surface of a-Ag 2À2x Ni x WO 4 microcrystals was also observed (Fig. 5(F)).  5(M and N)). In this case, higher concentrations of Ni added in the a-Ag 2-2x Ni x WO 4 structure are able to promote the growth of the (010) surface (displacement in the [010] direction) and yield its reduction (Fig. 5(O)).
The Wulff construction of the ideal structure, according to the surface energies previously obtained by our group, 17,40 and the morphology modulations due to the energy ratios are presented in Fig. 6. Calculations of the surface energy of different Ni substituted facets are performed and tabulated in Table S4 (ESI †), maintaining the ratio of surface energy between facets, compared to the pure system. According to our previous simulation, Ag 2 WO 4 under vacuum exhibits a cubic morphology. On this basis, considering the Wulff theorem, it is possible to change the surface energy ratios to find the experimental morphologies. This is a simple method to evaluate the energy ratios by circumventing all the real variables of an experimental system. As a result, a-Ag 2À2x Ni x WO 4 samples with x equal to 0 (Fig. 5(A-C)), 0.01 (Fig. 5(D-F)), and 0.02 (Fig. 5(G-I)) were composed mainly of particles with the same morphologies obtained theoretically with (110) E surf = 0.20 J m À2 and (110) E surf = 0.16 J m À2 /(011) E surf = 0.40 J m À2 (Fig. 6). The a-Ag 2À2x Ni x WO 4 sample with x = 0.04 and 0.08 (Fig. 5(J-L) and (M-O), respectively), although consisting of a few particles with morphologies similar to those of the previous samples, mainly showed cubic particles as the ideal system, and also (110) E surf = 0.26 J m À2 /(011) E surf = 0.37-0.40 J m À2 /(101) E surf = 0.53, as shown in Fig. 6. Another important factor to consider is the oscillation of the width observed in and between the samples. Fig. 6 shows one of the morphologies from a different angle, and the changes caused by the (001) E surf variation. The instability of the (001) surface in this model resulted in more elongated particles. Therefore, the oscillation of the elongation size in the experimental system produced evidence that the (001) surface had a considerable energy amplitude. Fig. 7 shows a schematic representation of the main stages involved in the synthesis and growth mechanisms of a-Ag 2À2x Ni x WO 4 microcrystals based on the FE-SEM micrographs.
Under hydrothermal conditions, the viscosity of water decreases, and the mobility of ions and dissolved molecules is then enhanced. The microwave energy acts directly on the rotation barrier of  the water molecules, forcing the kinetic crystallization of the particles. 48,53 Thus, the hydration energy is overcome by the strong electrostatic attraction between the Ag + /Ni 2+ and WO 4 2À ions, hence forming primary particles (Fig. 7(A and B)). As these nuclei have free rotation, they collide at random and rearrange in relation to each other through Brownian motion, 54 resulting in the formation of microcrystals with hexagonal faces. Moreover, microwave irradiation can promote the formation of aggregated particles under hydrothermal conditions. 55,56 When the system reached equilibrium between the solubility and precipitation processes, Ostwald ripening (OR) can occur 52 (Fig. 7(C and D)). In this mechanism, larger particles were formed by dissolving smaller particles, owing to the energy difference between them, reflected on their solubilities by the Gibbs-Thomson law. 57 In particular, the irregularities and surface defects found in a-Ag 2À2x Ni x WO 4 microcrystals can indicate the predominance of this type of crystal growth (Fig. 7(D)). The TEM images of the a-Ag 2 WO 4 and a-Ag 2À2x Ni x WO 4 (x = 0.08) microcrystals confirmed the dynamic aggregation of primary particles in various processes in different directions (Fig. S3 in the ESI †). The selected area of electron diffraction images in the insets of Fig. S3(A and B) (ESI †) for a-Ag 2 WO 4 and a-Ag 2À2x Ni x WO 4 (x = 0.08), respectively, reveal a common pattern of polycrystalline material electron diffraction.

Structural order at medium range
The optical band gap energy (E gap ) was calculated by the Wood and Tauc method, 58 using the function proposed by Kubelka and Munk. 59 According to Tang et al. 13 and Kim et al., 60 the Ag 2 WO 4 crystals exhibited an optical absorption spectrum governed by direct electronic transitions between the valence and conduction bands.
Theoretical results have shown that the symmetry breaking process, or the effect of the order-disorder transition in the structure of various semiconductors, is a necessary condition for the existence of intermediary energy levels within the forbidden band gap. 61  The computed band value for pure Ag 2 WO 4 was 2.0 eV. 1 When 6.25% of Ag was replaced by Ni, reduction to 0.81 eV and 1.29 eV was observed for the Ni substitutions of Ag2 and Ag4, respectively, as shown in Fig. S5 (ESI †). Notably, there was a greater decrease in the band gap when a major local distortion was caused by the Ni replacement (for Ag2, a change in the coordination occurred), in agreement with the experimental results.
In addition, calculations using this type of function typically underestimate the band gap values, typical for DFT calculations. 1,62 PL measurements were performed to provide a better understanding of the structural and electronic order-disorder effects at a medium distance in the a-Ag 2À2x Ni x WO 4 microcrystals. The PL spectra of a-Ag 2À2x Ni x WO 4 microcrystals revealed an intense blue luminescence emission (Fig. 8), with a broadband profile and a maximum emission of 454 nm located in the blue region. In the literature, the PL emission spectra of tungstates are frequently decomposed into violet, blue, green, yellow, orange, and red light components, 63 with some controversial interpretations with regard to the blue and green maximum PL emissions. [64][65][66][67] In general, it is assumed that the emission spectra of metal tungstates are associated with the charge-transfer transitions within the [WO 4 ] complexes, [68][69][70] or due to oxygen vacancies such as [WO 3 ÁV o ]. 64,71 The emission peaks detected at lower energies were related to deep level defects due to distortions in the lattice modifier clusters [AgO n ] (n = 2, 4, 6, and 7). 11 This behavior can also be observed in the PL spectra of other complex metal oxides involving different Ag clusters. 6,10,55,56 In the present materials, Ag 2À2x Ni x WO 4 (0 r x r 0.08), the PL emissions are in the green-orange region due to the substitution process of Ag by Ni atoms, owing to the formation of new [NiO n ] clusters in the Ag 2 WO 4 lattice, which were responsible for the different distributions and organizations of intermediate energy levels within the forbidden band gap. To better understand the influence of the different concentrations of Ni on the optical properties of the a-Ag 2À2x Ni x WO 4 solid solution, the individual contributions of each component of the PL spectra were decomposed using the Voigt function modeled in the PeakFit program (version 4.05), including the peak positions and corresponding areas, as shown in Fig. 9.
The literature 63 reports that deep holes are responsible for the green, yellow, orange, and red PL emissions at room temperature, while shallow holes cause the violet and blue emissions. Deconvolution of the PL spectrum of a-Ag 2 WO 4 , The cluster-to-cluster charge-transfer (CCCT) in a crystal is characterized by excitations involving electronic transitions from one cluster to another. 56  ] d clusters presented intrinsic defects, which were linked to the effects of the order-disorder transitions in the electronic structure, surfaces, and interfaces, and are commonly found in materials with a Scheelite-type structure and synthesized using the MAH method. 63,[72][73][74][75] The PL properties of Ag 2 WO 4 crystals could be explained considering the density of structural defects responsible for changes in the configuration of the intermediate states within the band gap. 2  . When the photon energy (l = 350 nm) is absorbed by the crystals, the processes represented by the notation of Kröger-Vink 76 may occur (eqn (1) to (4)): In general, the Ni atoms in a-Ag 2À2x Ni x WO 4 solid solutions lead to a more pronounced generation of shallow defects rather than deep holes, favoring the high-energy emissions (Fig. 9). The substitution of Ag by Ni atoms provokes the formation of more ordered clusters in the crystal lattice of the a-Ag 2À2x Ni x WO 4 (0.01 r x r 0.08) solid solutions. In the case of a-Ag 2À2x Ni x WO 4 (x = 0.04 and x = 0.08), the increased presence of [NiO n ] clusters seems to suppress PL emissions, as they interacted with the WO 6 ½ 0 o . AgO n ½ 0 o clusters, neutralizing them and contributing to the structural reorganization, as described by eqn (5) and (6).
In eqn (1) to (6), the superscript x indicates neutral clusters, indicates the cluster with one hole (h ), 0 indicates the cluster with one electron (e 0 ), and n equals 2, 4, 6, or 7 for the [AgO n ] cluster, while for the [NiO n ] cluster n is more likely to assume the values 4 and 7, considering the results of the Rietveld refinement.
In addition to the distortions in the [WO 6 ] and [AgO n ] clusters (n = 2, 4, 6, and 7), the PL of a-Ag 2À2x Ni x WO 4 microcrystals appeared to be associated with alterations in the morphology and aggregation of the particles. Then, the morphological aspects were able to dislocate the crystal lattice planes as well as adjacent particles in the aggregates, producing different density defects in the crystals. 56 In addition, changes in the shape and size of the particles can be considered as additional key factors responsible for the variations of the PL emission profiles. In this context, we can define the PL property as an average contribution of the surfaces and the core of the crystal, including all the defects. Therefore, it is possible to associate the changes in PL observed for the samples having different amounts of Ni with our morphology study described above (see also Fig. 5 and 6). Different exposed surfaces contribute as different PL sources, directly or indirectly. As concluded previously, the addition and increase of Ni content as a substituent in the system favored the formation of particles with a greater degree of symmetry, and consequently with a smaller quantity of different exposed surfaces. The correlation between FE-SEM images and theoretical morphologies is highlighted in Fig. S6 (ESI †). These results are in agreement with the narrowing of the PL band and also the decrease of deep defects, due to better homogenization of the PL sources.

Conclusions
In summary, a-Ag 2À2x Ni x WO 4 (0 r x r 0.08) solid solution crystals were successfully grown by the MAH method, and the introduction of Ni 2+ ions to simultaneously control the crystal structure, morphology, and optical properties was explored for the first time. FE-SEM, XRD, MR, FT-IR, TEM, UV-vis, and PL measurements were extensively employed to investigate the differences in the coordination environments and specific structural arrangements upon replacement of [AgO n ] Â by the [NiO n ] complex cluster. Theoretically, the presence of 6.25% of Ni that can be distributed at both Ag2 and Ag4 positions, which form clusters with coordination seven and four, respectively, indicated these as the most stable substitutions, in agreement with the Rietveld results. Furthermore, the changes observed in the PL behavior were associated with the presence of structural and electronic order-disorder effects, which caused a symmetry breaking and a local polarization in the crystal structure at medium range. This confirmed that the PL results are derived from a compromise between the presence of local defects (short-range disorder) and crystal lattice ordering (long-range order). Using Wulff's theorem, a simple model is suggested to determine the surface energies of a-Ag 2 WO 4 crystals with atomiclevel resolution, and the proposed computational technique was used to calculate the crystal morphologies at equilibrium. Potential particle shapes and their transformations in a-Ag 2 WO 4 were evaluated by controlling the ratio between surface energy values of each face using this model. The substitution of Ag by Ni decreases the number of exposed surfaces and the shape change from hexagonal to faceted cubic in a-Ag 2 WO 4 to a-Ag 2À2x Ni x WO 4 (x = 0.08), respectively. This induces an increased number of shallow defects, improving the blue contributions in the PL profile.
The present findings provide a better understanding of the morphological, structural, and optical properties of novel a-Ag 2À2x Ni x WO 4 microcrystals, and open the door for the modulation of their optical properties, and the development of novel devices based on new photonic materials.