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Chemistry-Inspired Adaptable Framework Structures Zhiguo Xia*,† and Kenneth R. Poeppelmeier*,‡ †

The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China ‡ Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208-3113, United States CONSPECTUS: Adaptable crystalline frameworks are important in modern solidstate chemistry as they are able to accommodate a wide range of elements, oxidation states, and stoichiometries. Owing to this ability, such adaptable framework structures are emerging as the prototypes for technologically important advanced functional materials. In this Account, the idea of cosubstitution is explored as a useful “pairing” concept that can potentially lead to the creation of many new members of one particular framework structure. Cosubstitution as practiced is the simultaneous replacement of two or more cations, anions, complex anions, other fundamental building units, or vacancies. Although the overall sum of the oxidation states is constant, each component is not necessarily isovalent. This methodology is typically inspired by either mineral-type structural prototypes found in nature or those discovered in the laboratory. Either path leads to the appearance of new phases and the discovery of new materials. In addition, the chemical cosubstitution approach can be successfully adopted to improve physical properties associated with structures. This Account is structured as follows: first, we illustrate the significance and background of chemical cosubstitution by reviewing mineral-inspired structures, such as perovskite and lyonsite, and the structural unit discovered in some selected solid state compounds. With time, the number of lyonsite related phases should rival or even surpass the perovskite family. Several members of the lyonsite-type have been identified as Li-ion conductors and photocatalysts. There is also a noncentrosymmetric structuretype, and therefore the other properties associated with the loss of inversion symmetry should be anticipated. Next, we illustrate recent advances in the synthesis of the new cosubstituted solid state materials from our two groups including (1) nonlinear optical materials, (2) luminescent materials, (3) transparent conducting oxides, and (4) photocatalyst and photovoltaic materials. We emphasize that a concerted and rigorous theoretical and experimental approach will be required to define thermodynamic stability of the complex cosubstitution chemistries, structures, and properties that are yet unknown. We conclude by summarizing the topic and suggesting other possible adaptable framework structures where cosubstitution can be expected.

1. INTRODUCTION Realizing unsuspected physical and chemical properties, predicting what stable structures would exhibit these desired properties, and finding ways to synthesize these materials is a daunting problem. The dual challenge currently facing materials-by-design is how the transformational potential of first-principle predictions can be achieved when, for these predictions to be meaningful, experimental feasibility is required.1−3 To innovate and discover new materials with optimal properties calls for sophisticated yet practical materials selection strategies.4,5 Suitable stable structures that are amenable to chemical substitution already exist and form a large class of technologically transformative materials. For example, perovskite (CaTiO3), garnet (Ca3Al2(SiO4)3), and spinel (MgAl2O4) are three examples of structures that exhibit large numbers of stable phases with different compositions.6−8 Properties can often be changed or even tuned by single element substitution. The situation is challenging, however, because the physical properties “living” in these compounds depend on the local composition and structure, and in complex multinary oxides, the variance in properties is to some extent magnified.9 © 2017 American Chemical Society

Clearly, adaptable framework structures play an important role in the discovery of new phases for different applications because the elemental makeup can be varied.10,11 Properties can generally be altered or modified based on particular elemental substitution patterns, which in turn provide sharp insight into the connection between structure and function. Therefore, the composition−structure−property paradigm has long been a key concept in solid state chemistry,12,13 and in our research cosubstitution is a very useful and versatile structure−property related design principle.14,15 Cosubstitution is the simultaneous replacement of two or more cations, anions, complex anions, fundamental building units, or vacancies. The overall sum of the oxidation states is constant, but each component is not necessarily isovalent. The steps involved in chemical cosubstitution-that led to the discovery of new solid state functional materials include the following procedures: (1) Selection of prototype phases with adaptable framework structures; (2) Design of new compounds via chemical cosubstitution; (3) Discovery of new solid state functional materials via rational Received: January 16, 2017 Published: April 25, 2017 1222

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Accounts of Chemical Research synthesis, phase analysis, and the final physical property characterization (Figure 1).

Figure 2. ABX3 perovskite-type compounds with A-cations (pink sphere) and BX6 corner-connected octahedra (blue) exhibit two typical transformations to form double perovskites (A2B′MeX6) by the cosubstitution of MeX6 + B′X6 = 2BX6. Several representative examples, CH3NH3PbI3, CsPbI3, Cs2AgBi(Br,Cl)6, and Cs2SnI6, are also listed in the table.

Figure 1. Schematic illustration of chemical cosubstitution-led discovery of new solid state functional materials. General steps include selection of an adaptable framework structure and design of new compounds by chemical cosubstitution, followed by synthesis and phase analysis.

modified to improve the moisture, irradiation, or heat stability and, in particular, to reduce the use of environmentally harmful elements such as lead by forming the stable quaternary A2B+Me3+X6 or A2□Me4+X6 double perovskites by chemical cosubstitution (see Figure 2). Recently, the new Cs2AgBi(Br,Cl)6 perovskite, which belongs to the Fm3̅m space group, was reported. It is constructed with BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure.23 Removing one B-site cation and replacing it with a vacancy yields a large family of vacancy-ordered double perovskites, A2BX6, as represented by Cs2SnI6 (see Figure 2). Such defectvariant perovskites adopt the K2PtCl6 structure type with isolated [BX6]2− octahedra coordinated by A+ cations. Cs2SnI6 adopts the cubic structure and exhibits intrinsic n-type electrical conductivity, air and moisture stability, and strong visible light absorption, all of which are advantageous properties for photovoltaic devices compared to those of CH3NH3PbI3. The example elucidates the important role of cosubstitution, in the preparation of not only new compounds but, more importantly, ones with improved or tunable properties compared to the initial prototype phase. Key criteria include the evaluation of the structural stability, both theoretically and experimentally, and ensuring a suitable band gap for these applications.24 A second but a lesser known mineral with similar adaptive properties takes its name from the mineral lyonsite, αCu3Fe4(VO4)6.11 To date, the lyonsite-type compounds have been mainly developed as ionic conduction and catalysis materials. Based on the composition and structure, the unit cell of a typical lyonsite-type oxide has the general formula A16B12O48. The descriptive chemical formula is AB2C(Me,Me′O4)3 (see Figure 3a) where A, B, and C are three independent cation sites occupied by +1, +2, +3, or +4 cations (Li+, Mg2+, Co2+, Cu2+, Mn2+, Fe2+, Fe3+, Zr4+, Hf4+, Ta4+, etc.) and Me and Me′ are +5 or +6 ions (Mo6+, W6+, and V5+). The large number of chemically diverse members is due to substitutional disorder of +1 to +4 cations on three distinct A, B, and C sites and additional disorder of +5 and + 6 cations on two sites Me and Me′. Many lyonsite-type compounds, such as Li2Cu2(MoO4)3 as ionic conductors or Mg2.5VMoO8 and Zn2.5VMoO8 as photocatalysts, have been reported. Through structural analysis and comparison of several representative and important members in this family, namely, Li2Fe2(MoO4)3, Li 3 Fe(MoO 4 ) 3 , Li 3 Ti 0.75 (MoO 4 ) 3 , Mg 2.5 VMoO 8 , and (Mg3.75(V0.5Mo0.5O4)3), several cosubstitution mechanisms are

Based on such a chemistry-inspired approach, our recent research has led to different new optical functional materials, namely, nonlinear optical materials, transparent conducting oxides, luminescent materials, and photocatalyst and photovoltaic materials. In this Account, we will summarize research from our two groups that illustrates the use of various structures and cosubstitution with the objective of fusing understanding of crystal chemistry to the power of modern computational methods.16,17

2. COSUBSTITUTION DESIGN TOWARD NEW PHASES WITH ADAPATABLE FRAMEWORK STRUCTURES 2.1. Mineral-Inspired Discovery of New Phases by Cosubstitution

Minerals represent a significant portion of our modern scientific databases. Many minerals exhibit adaptable framework structures. These natural materials from the mineral kingdom have a long and exciting history, have had significant impact on the laboratory bench,18,19 and have led to the formation of many inorganic phases with tunable physical properties. Perovskite and lyonsite are two typical examples. Perovskites are among the most fascinating mineral crystals and have earned the distinction of “chemical chameleons”, given that over 50% of all elements can be incorporated on at least one of the three lattice sites of the general ABX3 stoichiometry.20,21 Moreover, perovskites play important roles in a variety of modern day applications. Lead-based organohalide perovskites have emerged recently as arguably the most promising solution-based solar cell materials or quantum-dot light-emitting diodes.21 ABX3 organohalides or all-inorganic halide perovskites, where A is an alkyl ammonium cation (CH3NH3+) or inorganic cations (Cs), B is Pb2+ or Sn2+, and X is a halide ion (I−, Br−, Cl−), have made possible these rapid and remarkable advancements in part because of their unique and adaptable crystal structure. Cosubstitution at all three sites provides many opportunities to discover other new phases with diverse compositional and structural diversity.22,23 Figure 2 depicts two typical ways to form the double perovskites (A2B′MeX6) by cosubstitution. In this example, the simple metal-halide perovskites CH3NH3PbI3 or CsPbI3 can be 1223

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Figure 4. Substitution of the Ba2+ ions in α-BaB2O4 (α-BBO) with cosubstituted [Na3F]2+ units leads to Na3Ba2(B3O6)2F (NBBF). Adapted with permission from ref 31. Copyright 2015 American Chemical Society.

Figure 3. Descriptive chemical formula for lyonsite-type oxides AB2C(Me,Me′O4)3 (a) and four representative members of this family, Li3Fe(MoO4)3, Li2Fe2(MoO4)3, Li3Ti0.75(MoO4)3, and Mg2.5VMoO8 (Mg3.75(V0.5Mo0.5O4)3) (b). For each compound, the filled circles with labels illustrate the distribution of cations on the A, B, C, Me, and Me′ sites. Arrows with numbers in panel b depict six proposed cosubstitution mechanisms as listed in panel c.

However, a twist of the [B3O6]∞ network leads to a phase transition in α-BBO and a structural instability. Therefore, through the design of the cosubstituted [Na3F]2+ unit, which substitutes for the Ba2+ ion, NBBF can be formed and the intrinsic phase transition is eliminated. The desired structural features of the [Ba2B6O12]∞ double layers (Figure 4), and the connection of these adjacent layers is strongly reinforced through the new Na−O bonds and Ba−F bonds. The imbalance of Lewis acidity within the cationic framework of α-BBO has been removed by the addition of the [Na3F]2+ unit. Clearly, structural unit design based on the concept of cosubstitution opens up many pathways to modify the properties of solid state materials when a large imbalance in Lewis acidity triggers structural instabilities and phase transitions.

discernible, as shown in Figure 3b,c.25,26 Generally, they can be further classified into two types: (1) cosubstitution over A, B, and C cation framework only (types 1, 2 and 3 in Figure 3c) and (2) enlarged cosubstitution involved A, B, and C and Me and Me′ cation frameworks (types 4, 5, and 6 in Figure 3c). Moreover, they can also be classified into two other types: (1) without formation of vacancies (type 1 and type 4) and (2) with formation of vacancies (type 2, type 3, type 5, and type 6). These possible mechanisms may likely lead to numerous other stable lyonsite configurations of cations and stoichiometries, and their total number may far exceed other structure types, including perovskite. Discoveries of new phases based on complex cosubstitution patterns have made great progress in recent years, in part because of advances in both single crystal and powder X-ray and neutron diffraction. Many other mineral types, such as garnet, apatite, melilite, fluorite, and wurtzite, will likely yield a large number of new materials and phenomena.

3. DISCOVERY OF NEW SOLID STATE MATERIALS WITH ADAPATABLE FRAMEWORK STRUCTURES 3.1. Discovering New Nonlinear Optical Materials by Cosubstitution

SHG is the frequency doubling of light. Because lasers can only be generated in specific wavelengths, SHG is useful for generating lasers of higher energy, including the ultraviolet (UV) region. As light travels through a material, the latter is polarized through the interaction of electrons with the light. In a SHG-active material, the polarization contains nonlinear components; it can be expressed as a Taylor expansion with a nonzero squared component. The efficiency of SHG is determined by the magnitude of the coefficient associated with squared term, usually defined as χ2. This represents the SHG susceptibility of a given material. The same principle can be expanded for higher order harmonic generation as well. Noncentrosymmetry is a prerequisite for SHG since χ2 = 0 for all centrosymmetric structures.32 Accordingly, a number of NLO materials have been discovered, and several have been developed for commercial use covering the ultraviolet to infrared spectral regions.33 Moreover, many efforts have been made to resolve the relationship among the composition, structure, and NLO properties of materials. It has been found that some interesting structural units in different adaptable structures play a key role in the discovery of stable new phases with moderate SHG coefficients, large birefringence, viable crystal growth habit, and so on.34 Nevertheless, it still remains challenging to efficiently design a suitable high-performing NLO material by proper design principles. Professor Chen’s Anionic Group Theory has established that efficient SHG-active crystals often have anions with aligned polar moments in the solid state.35 Chemical cosubstitution was recently used to

2.2. Structural Unit Design Based on Cosubstitution

Mineral-inspired discovery of new phases mentioned above has led to the expansion of our chemical databases, more thorough screening for desired properties, and accelerated materials innovation. In addition to these examples from the mineralogy literature, chemical substitution of a structural unit, which is formulated based on cosubstitution, can lead to the appearance of a new phase with new properties. It is well-known that the structures of many adaptable crystalline frameworks can be deconstructed into a set of smaller and simpler building modules.27,28 Therefore, numerous structural units and a wide variety of possible combinations exist for various adaptable structures. As an example, such a strategy has proved effective in the discovery of new nonlinear optical (NLO) materials.29,30 The structural unit (Be2BO3F2)∞ layer in KBe2BO3F2 (KBBF) determines the relatively large second-harmonic generation (SHG) coefficient, a short absorption edge, and moderate birefringence. The layers are made up of trigonal planar [BO3] and tetrahedral [BeO3F] units. Figure 4 shows the crystal structures of α-BaB2O4 (α-BBO) and Na3Ba2(B3O6)2F (NBBF) highlighting the modification of the latter by Ba2+ ↔ [Na3F]2+.31 As shown in Figure 4, the basic building units of α-BBO are parallel planar B3O6 anionic groups, which enhance the anisotropic polarizability to produce a large birefringence. 1224

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new NLO material, K3Ba3Li2Al4B6O20F, has been designed by cosubstituting 2Be2+ = [Li+ + Al3+] and substitution of K+ for Na+ and Ba2+ for Ca2+.37 This compound features a doublelayered structure that preserves the structural merits of SBBO. Thus, it exhibits no layering growth tendency and possesses the optical properties required for deep-UV NLO applications. It also provides a new opportunity to design beryllium-free deepUV NLO materials with good performance. Moreover, the substitution of Zn for Be can also be one strategy to obtain new members of KBBF family, as demonstrated by Cs3Zn6B9O21 with large SHG response.33 Chemical cosubstitution strategies have led to other new NLO materials. Recently, the (BO3F)4−, (BO2F2)3− and (BOF3)2− groups have been shown to be new functional units that can produce a large birefringence without layering tendency and simultaneously keep a short cutoff edge down to deep-UV.38 As reported by Pan’s group, by [O2−] = [2F−] in B−O anionic groups, the new beryllium-free material design and synthesis strategy for deep-UV NLO materials has been proposed, which mentions that LiB3O5 can evolve to Li2B6O9F2. In collaboration with Professor Pan, our group demonstrated another example based on the report of K3B6O10Cl (KBOC) with an inverse perovskite-related structure.39 As shown in Figure 6, the structures of perovskite

produce some strong IR NLO materials. However, a different phrase, “Dual Ion Substitution Synergy”, was adopted to express the intrinsic character of “cosubsitution”.36 Based on a direct SHG process, the important NLO material KBBF generates deep-UV coherent light. It has a large response coefficient, a short absorption edge, and sufficient birefringence.37 However, its interlayer bonding is dominated by K−F ionic bonds and a strong layering tendency occurs in the process of crystal growth. This severely limits the power of coherent laser output. To overcome the crystal-growth problems and maintain all of the optical properties of the KBBF crystal, Sr2Be2B2O7 (SBBO) has been designed and synthesized.29 As shown in Figure 5a, the chemical cosub-

Figure 6. Partial crystal structures of typical perovskite CaTiO3 (a) and the NLO material K3B6O10Cl (KBOC), highlighting the design principle via the proposed cosubstitution strategy, [Ca2+ + Ti4+ + O2−] = [[B6O10]2− + Cl− + K+], and the crystal chemistry formula can be written as [B6O10]ClK3 for KBOC. Adapted with permission from ref 39. Copyright 2011 American Chemical Society.

Figure 5. Structural evolution in several NLO materials: (a) from KBe2BO3F2 (KBBF) to Sr2Be2B2O7 (SBBO) via the cosubstitution of [K+ + F−] = [Sr2+ + O2−], (b) phase construction of NaCaBe2B2O6F via the cosubstitution of [O2− + Sr2+] = [F− + Na+] and simultaneous cation substitution of Ca2+ for Sr2+, and (c) new phase of K3Ba3Li2Al4B6O20F from the synergistic effect of cosubstitution of 2Be2+ = [Li+ + Al3+] and substitution of K+ for Na+ and Ba2+ for Ca2+. Adapted with permission from ref 37. Copyright 2016 American Chemical Society.

CaTiO3 and the NLO material KBOC (noncentrosymmetric and rhombohedral space group R3m) are related, that is, KBOC consists of the A-site hexaborate [B6O10] groups and the BX3 chloride-centered octahedral [ClK6] groups linked together through vertices to form the perovskite framework represented by ABX3. The design principle via the proposed cosubstitution strategy, [Ca2+ + Ti4+ + O2−] = [[B6O10]2− + Cl− + K+] leads to KBOC. That is to say, the positions of Ti and O are similar to those of Cl and K, respectively. As a result, their formulas can be written as CaTiO3 and [B6O10]ClK3, respectively, as shown in Figure 6b. In addition to the borates, chemical cosubstitution at different sites can also modify the NLO properties of other systems. For example, with the replacement of NbO6 by TiO5F (Cosubstitution of Nb5+ + O2− = Ti4+ + F−), the SHG efficiency of as-designed Pb2TiIVOF(SeO3)2Cl is much larger than that of Pb2NbVO2(SeO3)2Cl.40 Mao’s group also reported the first bismuth selenite fluoride, BiFSeO3, originating from the chemical cosubstitution of BiOIO3 via the F− + Se2− = O2− + I−.41 BiOIO3 with a high SHG efficiency of 12.5 times that of KDP was chosen as the prototype compound. Cosubstitution of iodate and oxo-anions by selenite and fluoride anions led to

stitution of [K+ + F−] = [Sr2+ + O2−] has been applied to enable the structural transformation from KBBF to SBBO. Based on such a structural design in this adaptable framework, the structure of SBBO possesses a nearly planar infinite [Be3B3O6]∞ network perpendicular to the c axis, providing large SHG coefficients and sufficient birefringence. More importantly, the binding between the [Be3B3O6] layers is stronger, as they are bridged by O atoms bound to Be atoms, which can benefit crystal growth. SBBO was then regarded as one of the most promising candidates for the next generation of deep-UV materials. However, large SBBO crystals with high optical quality have not been obtained yet. Therefore, as shown in Figure 5b, the cosubstitution of [O2− + Sr2+] = [F− + Na+] has been applied and NaCaBe2B2O6F was discovered.30 As compared with SBBO, NaCaBe2B2O6F possesses a larger space to accommodate the even smaller Na+ cations inside its [Be2B2O6F]∞ double-layers, which in turn stabilize its crystal structure. However, NaCaBe2B2O6F exhibits a small SHG response, and the constituent beryllium is highly toxic. Another 1225

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and M4 are all the possible local coordination environments for the luminescence center Eu2+ (Figure 7b). The observed fourtype coordinated structures over the evolution is responsible for a three-stage spectral shift of the emission spectra. First is the observed blue-shift, which shifted significantly from 663 to 738 nm and, finally, there is a sudden shift to 600 nm when x is 1.45 Clearly, this cosubstitution methodology leads to new phases and tunes the properties of phosphors. Recently, the isostructural solid solutions of (Ca1−xLax)(Al1+xSi1−x)N3:Eu2+ and (Ca1−xLix)(Al1−xSi1+x)N3:Eu2+ have been designed by the cosubstitution of [La3+−Al3+] for [Ca2+−Si4+] and [Li+−Si4+] for [Ca2+−Al3+] in the host/activator CaAlSiN3:Eu2+ system. There is a remote control effect that guides Eu2+ activators in selective Ca2+ sites that can efficiently tune optical properties. This is especially true regarding thermal stability, which is sensitive to valence variation in local structures and will act as a general strategy to luminescent materials.46 In most cases, realization of isostructural phases plays an important role in photoluminescence tuning. However, this cosubstitution strategy can be used even if the isostructural crystalline phase cannot be obtained, but tunable emission originating from a different mechanism can be obtained. Figure 8a gives the example of the photoluminescence tuning in

the discovery of BiFSeO3 exhibiting a SHG response of about 13.5 times that of KDP. Recently, two other metal iodate fluorides, BiF2(IO3)42 and Bi3OF3(IO3)4,43 with excellent NLO properties were obtained via the cosubstitution of O2− with 2F−. 3.2. Discovering New Luminescent Materials by Cosubstitution

Inorganic solid state luminescent materials, also called phosphors, are usually designed from selected hosts and activators. At present, there have been extensive studies in the discovery of the multicolor-emitting phosphors for different illumination and display applications represented by the white LEDs. Modification of the chemical compositions by cosubstitution of the host plays an important role in the photoluminescence tuning and the luminescence optimization of the phosphors.7 Recently, we carried out several cosubstitution studies, including experimental and computational investigations on the solid-solution phosphors Ca2(Al1−xMgx)(Al1−xSi1+x)O7:Eu2+.15 As shown in Figure 7a, it depicts the

Figure 7. (a) Representative diagram illustrating the chemical cosubstitution design [Mg2+−Si4+] for [Al3+−Al3+] in the primary phase Ca2Al(AlSiO7) (x = 0, S1) to [Ca2Mg(Si2O7) (x = 1, S5) to produce the phosphor hosts Ca2(Al1−xMgx) (Al1−xSi1+x)O7. (b) Structural evolution model for the cosubstitution [Ca2+ + Al3+] = [Li+ + Si4+] to form the solid solutions (Ca1−xLix) (Al1−xSi1+x)N3. Adapted with permission from refs 44 and 45. Copyright 2015 and 2016 American Chemical Society.

proposed chemical unit cosubstitution of [Mg2+−Si4+] for [Al 3+ −Al 3+], where the cosubstitution is restricted to tetrahedral sites, and the structural evolution from S1, Ca2Al(AlSiO7), to S5, Ca2Mg(Si2O7), appears with the interval of x = 0.25. Such a cosubstitution leads to the phase formation of a series of isostructural compounds and enables the tuning of photoluminescence. The emission peaks shift from 513 to 538 nm, as simulated by the crystal-field model. Except for oxidebased phosphors, oxy-nitride or nitride phosphors can be also designed based on this cosubstitution strategy, such as the cosubstitution of [B3+−O2−] by [Si4+−N3−] from La5Si2BO13 to La5Si3O12N with the phase formation of La5(Si2+xB1−x)(O13−xNx):Ce3+ phosphors,44 and the cosubstitution via the [Ca2+ + Al3+] by [Li+ + Si4+] to form the (Ca1−xLix)(Al1−xSi1+x)N3 solid solutions.45 As shown in Figure 7b, it demonstrated the structural evolution model from CaAlSiN3 to LiSi2N3. When x is 0, CaAlSiN3 has a relatively stable and rigid crystal structure, and the coordinated tetrahedrons in the sixring of Ca2+ ions contain three [AlN4] tetrahedrons and three [SiN4] tetrahedra (labeled as M1 in Figure 7b). Based on the cosubstitution of [Ca2+ + Al3+] by [Li+ + Si4+], the local structures of the Eu2+ luminescence centers appear as three models: two [AlN4] tetrahedra and four [SiN4] tetrahedra (M2), one [AlN4] tetrahedron and five [SiN4] tetrahedra (M3), and six [SiN4] tetrahedra (M4) (when x = 1). M1, M2, M3,

Figure 8. Photoluminescence spectra a function of x in (CaxNa1−x)(MgxSc1−x)Si2O6:Eu2+ phosphors showing the cosubstitution of [Ca2+ + Mg2+] = [Na+ + Sc3+] (a) and Sr1−xY0.98+xCe0.02Si4N7−xCx phosphors with the cosubstitution of [Sr2+−N3−] for [Y3+−C4−] unit (b). Adapted with permission from refs 47 and 48. Copyright 2016 American Chemical Society and 2013 Wiley-VCH.

(CaxNa1−x)(MgxSc1−x)Si2O6:Eu2+, and hosts were constructed by the cosubstitution of [Ca2+−Mg2+] by [Na+−Sc3+] from CaMgSi2O6 to NaScSi2O6.47 As shown in the emission spectra as a function of x (Figure 8a), the emission centers and full width at half-maximum values remain invariant, indicating two unique, independent spectroscopic signatures. In fact, this is the first example of photoluminescence tuning induced by the cation segregation via spinodal decomposition, which is ascribed to the formation of the CaMgSi2O6:Eu2+ and NaScSi2O6:Eu2+ nanodomains and the corresponding two Eu2+ centers.47 Figure 8b gives another example on emission spectra evolution of the Sr1−xY0.98+xCe0.02Si4N7−xCx phosphors through segregation of activators on the nanoscale.48 Herein, the structures types of the two end-members are different. SrYSi4N7-type structure contains a network of corner linked N(SiN3)4 structural units with hexagonal (space group P63mc) symmetry, while Y2Si4N6C-type structure has monoclinic P21/c symmetry with C substituted only at the 4-connected positions 1226

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each other (Figure 9c) in order to show relation to another transformation mechanism (Figure 9d). Such blocks (SnO6− ZnO6) are distributed randomly, and the “average” crystal structure has the same symmetry and unit cell. The corundumtype In2−2xSnxZnxO3 solid solution (cor-ZITO, x ≤ 0.7) was prepared under a high pressure, that is, the cor-ZITO of a highpressure polymorph of the TCOs bixbyite, In2−2xSnxZnxO3 (x ≤ 0.4). Analysis of the extended X-ray absorption fine structure suggests that significant face-sharing of Zn and Sn octahedra occurs, and Zn and Sn are displaced and form oxygen bonds with lengths that are similar to those observed in high-pressure ZnSnO3.50 The formation of O2− vacancies: 2In3+ + O2− = 2Me12+, for example, (InO6−InO6) block changes by (ZnO5−ZnO5) (Figure 9a,c) and one O2− ion is released, is a form of cosubstitution.51 These (ZnO5−ZnO5) blocks are not very concentrated and are distributed randomly throughout the crystal. It should be noted that earlier mentioned mechanism 2In3+ = Me12+ + Me24+ can be followed by 2In3+ + O2− = 2Me12+ transformation and a variety of cosubstituted situations are potentially possible, for example, In2−x−ySnxZnyO3−(y−x)/2 (Figure 9c,d). With indium as a majority of the cation content, cosubstituted compositions readily adopt anion-deficient fluorite related structures, and delving deeper into their properties continues.52

to form C(SiN3)4 units. Although two end members belong to different structure types, they possess the same general formula as A2B4X7, which is 1147-type for SrYSi4N7 and 2461-type for Y2Si4N6C. Then, Sr1−xY0.98+xCe0.02Si4N7−xCx phosphors were obtained based on chemical cosubstitution via the [Sr2+−N3−] for [Y3+−C4−] unit. As chemical composition x increases from 0 to 1, the peak positions of both the excitation and emission spectra of Sr1−xY0.98+xCe0.02Si4N7−xCx phosphors show an obvious red-shift, so that tunable blue-yellow emission can be realized (Figure 8b). 3.3. Discovering New Transparent Conducting Oxides by Cosubstitution

Transparency to visible light and electrical conductivity appear at first glance to be mutually exclusive, after all highly conductive materials such as copper are typically opaque, while transparent materials such as glass are often electrically insulating. The ideal transparent conducting oxides (TCOs) would be fully transparent in a wide range of wavelengths, as well as have metal-like conduction properties. TCOs are fundamental components in optoelectronic devices such as the transparent electrode, a necessary component for flat panel displays and photovoltaic devices. Among them, ITO (tindoped indium oxide) has a typical conductivity of (1−5) × 103 S/cm and a transparency of 85−90% in thin films.14 Although ITO meets the needs of current devices, research is ongoing to develop new TCOs with a reduced amount of expensive indium element. One direction that leads to TCOs with less indium depends on chemical cosubstitution, a strategy based on In2O3 that has been shown to be effective.49,50 This rationale has successfully produced TCOs in MxIn2−2xSnxO3, where M = Mg, Ca, Zn, Cu, Ni, or Cd and In2−xX2x/3Sbx/3O3, where x = Zn or Cu, as shown in Figure 9.

3.4. Discovering New Photocatalyst and Photovoltaic Materials by Cosubstitution

Visible light driven photocatalysts for solar hydrogen production from water have been studied extensively. A number of photocatalysts have been proposed with sufficiently small band gap and appropriate band gap position for water splitting. Cosubstitution in adaptable structures is one avenue that can lead to photocatalysts with tunable band gaps and modified structural and chemical properties.53 As mentioned already, cosubstitution is a common tool in the chemist’s toolbox. For example, a solid solution of gallium nitride (GaN) and zinc oxide (ZnO) can be regarded as the cosubstitution of [Ga3+ + N3−] by [Zn2+ + O2−] unit.53 GaN possesses a band gap of ∼3.4 eV, and it is verified to adapt for overall water splitting under UV light. ZnO is also a famous wide-bandgap semiconductor of the II−VI group with a band gap of 3.2 eV. Since both materials have wurtzite structures with similar lattice parameters (GaN, a = b = 3.19 Å, c = 5.19 Å; ZnO, a = b = 3.25 Å, c = 5.21 Å), a solid solution of (Ga1−xZnx)(N1−xOx) can be expected with visible light response. Two typical semiconductors, ZnS with wide bandgap and AgInS2 or CuInS2 with narrow bandgap, are similar in crystal structure. The energy gap of ZnS−AgInS2 and ZnS−CuInS2 solid solutions could be modified in a wide range of visible light by changing the chemical composition, and the resulting phases exhibited broad PL spectra with tunable emission from green to red accordingly. 54 Moreover, Cu 2 ZnSnS 4 (CZTS) or Cu2ZnSnSe4 (CZTSe) are among the quaternary I2−II−IV− VI4 compounds and indium-free photovoltaic materials. In fact, for the Cu(I)2−Zn(II)−IV−VI4 compounds, there are three basic crystal structures, that is, kesterite (KS)-, stannite (ST)-, and wurtz-stannite (WST)-type. Phase relationship among them can be considered as the typical cosubstitution routes from ZnS to CuInS2 and then to Cu2ZnSnS4.55 As shown in Figure 10, the crystal structures of ZnS, CuInS2, and Cu2ZnSnS4 and their structural evolution pathways have been clearly demonstrated. The cosubstitution mechanism of 2Zn2+

Figure 9. Crystal structure of several TCOs from the prototype phase In2O3 (a) to new phases of In2−xZnxO3−x/2 (b), In2−2xSnxZnxO3 (c) and In2−x−ySnxZnyO3‑(y−x)/2 (d). The two typical cosubstitution pathways, 2In3+ + O2− = 2Zn2+ and 2In3+ = Zn2+ + Sn4+, are highlighted in this figure.

One of the simplest ways to transform In2O3 is the 2In3+ = Me12+ + Me24+ mechanism (Figure 9a,c). We will consider Me12+ = Zn2+, Me24+ = Sn4+ for simplicity. Two In3+ ions are replaced by two other ions with both smaller and larger charges, where the sum over the unit cell remains zero. Locally the Zn2+, Sn4+ ions break translation symmetry on the cation sublattice and (SnO6−ZnO6) block appears instead of (InO6−InO6) in some unit cells. Two (SnO6−ZnO6) blocks are shown close to 1227

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complex multiple cosubstitution methods can be inspired by numerous examples found in nature or new structures first discovered in the laboratory. Several examples were given, including well-known perovskite-type phases or the less wellknown lyonsite-type phases. Chemical cosubstitution can be readily applied to extend and modify properties of solid state compounds, including the nonlinear optical materials, luminescent materials, transparent conducting oxides, and photocatalytic and photovoltaic materials. There are many other examples in other fields of materials science, such as the design of the new cathode materials for lithium ion batteries, superconductor materials, ferroelectric and dielectric materials, thermoelectric materials, magnetic materials, and so on, which are expected to have promising physical properties. Despite all the above success, it should be emphasized that more theoretical and experimental research is needed to understand the intrinsic substitution mechanism on the thermodynamic stability of the crystalline phase and the effect of these stable local structures on the physical properties.

Figure 10. (a) Structural evolution from ZnS to CuInS2 via the chemical cosubstitution of 2Zn2+ = Cu+ + In3+. (b) Structural evolution from CuInS2 to Cu2ZnSnS4 via the chemical cosubstitution of 2In3+ = Zn2+ + Sn4+.

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= Me1+ + Me23+ (Me12+ = Cu+, Me24+ = In3+ for simplicity) in ZnS (Figure 10a) does not lead to the formation of cation or anion vacancies. But it is interesting that the addition of Cu+ and In3+ leads to a new ordering vector at the (1/2, 1, 0) k8point (W) of the Brillouin zone of the F4̅3m unit cell unit. Such symmetry breaking enables the space group change F4̅3m → I4̅2d, and the Zn site splits into the two new sites Zn1 and Zn2 with 4a and 4b Wyckoff sites, respectively. Cu+ and In3+ ions are not randomly distributed over these two sites, Cu+ ions prefer Zn1 site, but In3+ ions occupy Zn2 site (Figure 10a). Furthermore, the existence of Me23+ in the substituted final structure Me1Me2S2 implies that another mechanism can be also applied: 2Me13+ = Me32+ + Me44+ (in our case Me13+ = In3+, Me32+ = Zn2+, Me44+ = Sn4+) (Figure 10b). In this case, some symmetry elements disappear and the space group changes as I4̅2d → I4̅2m. The Zn1 site that was occupied by Cu+ and Zn2 site occupied by In3+ are now split into the three sites Zn1′, Zn2′, and Zn3′ with 2a, 2b, and 4d Wyckoff notations, respectively. Sn4+ ions occupy only the Zn2′ site, and Zn2+ occupy only Zn3′ site. Because the number of Zn3′ sites in the unit cell is twice that of Zn2′ sites (4d/2b), now equal amounts of Sn4+ and Zn2+ ions instead of two In3+ leads to full occupation of Zn2′ sites by Sn4+. However, only half of Zn3′ sites are occupied by Zn2+ ions (Figure 10b). The remaining half of Zn3′ sites and full Zn1′ sites are occupied by Cu+ ions, and then the original ordered structure becomes disordered. Therefore, through substituting Cu by other group I (+1 valence) cations, Zn by group II (+2 valence) cations, Sn by group IV (+4 valence) cations, and S or Se by group VI anions (−2 valence), a class of I2−II−IV−VI4 quaternary semiconductors can be designed, which have tetrahedrally coordinated crystal structures and electronic structure (s−p band gap) similar to the binary II−VI semiconductors (the prototype phase ZnS). Accordingly, versatile properties can be realized, and depending on the different compositions, there are wide applications in electronic, optoelectronic, thermoelectric, photovoltaic, or photocatalytic materials.56,57

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z. Xia). *E-mail: [email protected] (K. R. Poeppelmeier). ORCID

Zhiguo Xia: 0000-0002-9670-3223 Notes

The authors declare no competing financial interest. Biographies Zhiguo Xia is the professor of materials chemistry and physics in University of Science and Technology Beijing (USTB). He received his Ph.D. degree (Inorganic Chemistry) from Tsinghua University (Beijing, China) in 2008. After graduation, he has been working as an assistant and associate professor in China University of Geosciences, Beijing; then he joined USTB as a full professor. His research focuses on inorganic solid state chemistry, especially the designing of new rare earth phosphors for white LED by integrating structural discovery, modification, and structure−property relations. Kenneth R. Poeppelmeier is the Charles E. & Emma H. Morrison Professor in the Department of Chemistry and the Director of the Center for Catalysis and Surface Science at Northwestern University. His research covers several subjects in the field of inorganic solid-state chemistry, including the growth of single crystals and the synthesis of new transparent conductors, which emphasizes the connections between the synthesis and structure of new materials, the physical properties of new materials, and the technological advances that result from these discoveries.

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ACKNOWLEDGMENTS We firstly thank our students and collaborators for their efforts in this project over the years. The work was supported by the National Natural Science Foundation of China (Grant Nos. 91622125 and 51572023) and Natural Science Foundations of Beijing (2172036). K.R.P. recognizes that this work was made possible by support from the National Science Foundation (Awards DMR-1608218 and DMR-1307698 and a Graduate Research Fellowship under Grant No. DGE-1324585).

4. CONCLUSIONS AND PERSPECTIVES This Account focused on the useful and readily applied concept of cosubstitution and the wide variety of adaptable framework structures that can exhibit simple to complex cosubstitution chemistries. Various approaches involving simple or more 1228

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