Cubic-tetragonal Phase Transition Research Articles

Growth of Single Crystals of (K1-xNax)NbO3 by the Self-Flux Method and Characterization of Their Phase Transitions.

In this study, single crystals of (K1-xNax)NbO3 are grown by the self-flux crystal growth method and their phase transitions are studied using a combination of Raman scattering and impedance spectroscopy. X-ray diffraction shows that single crystals have a perovskite structure with monoclinic symmetry. Single crystal X-ray diffraction shows that single crystals have monoclinic symmetry at room temperature with space group P1211. Electron probe microanalysis shows that single crystals are Na-rich and A-site deficient. Temperature-controlled Raman scattering shows that low temperature monoclinic-monoclinic, monoclinic-tetragonal and tetragonal-cubic phase transitions take place at -20 °C, 220 °C and 440 °C. Dielectric property measurements show that single crystals behave as a normal ferroelectric material. Relative or inverse relative permittivity peaks at ~-10 °C, ~230 °C and ~450 °C with hysteresis correspond to the low temperature monoclinic-monoclinic, monoclinic-tetragonal and tetragonal-cubic phase transitions, respectively, consistent with the Raman scattering results. A conduction mechanism with activation energies of about 0.5-0.7 eV was found in the paraelectric phase. Single crystals show polarization-electric field hysteresis loops of a lossy normal ferroelectric. The combination of Raman scattering and impedance spectroscopy is effective in determining the phase transition temperatures of (K1-xNax)NbO3.

Stabilizing Crystal Framework of an Overlithiated Li1+xMn2O4 Cathode by Heterointerfacial Epitaxial Strain for High-Performance Microbatteries.

To meet the increasing demands of high-energy and high-power-density lithium-ion microbatteries, overlithiated Li1+xMn2O4 (0 ≤ x ≤ 1) is an attractive cathode candidate due to the high theoretical capacity of 296 mAh g-1 and the interconnected lithium-ion diffusion pathways. However, overlithiation triggers the irreversible cubic-tetragonal phase transition due to Jahn-Teller distortion, causing rapid capacity degradation. In contrast to conventional lithium-ion batteries, microbatteries offer the opportunity to develop specific thin-film-based modification strategies. Here, heterointerfacial lattice strain is proposed to stabilize the spinel crystal framework of an overlithiated Li1+xMn2O4 (LMO) cathode by epitaxial thin film growth on an underlying SrRuO3 (SRO) electronic conductor layer. It is demonstrated that the lattice misfit at the LMO/SRO heterointerface results in an in-plane epitaxial constraint in the full LMO film. This suppresses the lattice expansion during overlithiation that typically occurs in the in-plane direction. It is proposed by density functional theory modeling that the epitaxial constraint can accommodate the internal lattice stress originating from the cubic-tetragonal transition during overlithiation. As a result, a doubling of the capacity is achieved by reversibly intercalating a second lithium ion in a LiMn2O4 epitaxial cathode with a complete reversible phase transition. An impressive cycling stability can be obtained with reversible capacity retentions of above 90.3 and 77.4% for the 4 and 3 V range, respectively. This provides an effective strategy toward a stable overlithiated Li1+xMn2O4 epitaxial cathode for high-performance microbatteries.

Enhanced electrocaloric effect in Sr-doped Ba0.85Ca0.15Zr0.1Ti0.9O3 lead-free thin films near room temperature

Ferroelectric thin films with giant room-temperature electrocaloric effect (ECE) could be very useful in design of solid-state refrigeration devices. Here, lead-free Ba0.85Ca0.15-xSrxZr0.1Ti0.9O3 (BCZT-xSr, 0.01 ≤ x ≤ 0.10) thin films are successfully prepared on the substrates (Pt/Ti/SiO2/Si) via sol-gel route, and the effect of the Sr substitution on the structure, electric properties and ECE of the films were investigated. The dielectric analyses show that the rhombohedral-tetragonal phase transition temperature (TR-T) move to a higher temperature and the tetragonal-cubic phase transition temperature (Tm) shift to a lower temperature with increasing Sr content, and finally the TR-T peaks merge into the Tm peaks at room temperature. The BCZT-0.05Sr film presents a giant ECE with ΔT = 27.1 K and ΔS = 30.6 J K−1 kg−1 at 800 kV cm-1 near room temperature (301 K). The giant ΔT benefits from a multiphase coexisting region with successive R-T and T-C phase transition character near room temperature. These results indicate that the lead-free Sr-doped BCZT thin film is an attractive material for application in ECE refrigeration devices.

Stability of ferroelectric phase and structural characteristics in oriented PbTiO3 ceramic coating formed by aerosol deposition method

Temperature-dependent structural variations of ceramic coatings of lead titanate PbTiO3 (PT) formed at room temperature (RT) via aerosol deposition (AD) have been investigated to evaluate the stability of the ferroelectric phase. Synchrotron radiation x-ray diffraction experiments in the as-deposited state showed a polycrystalline film oriented preferentially along the c-axis of the perovskite-type ferroelectric tetragonal structure on the quartz glass substrate. The c-axis orientation may be attributed to the anisotropic structural characteristics of the crystal with a layered structure composed of a two-dimensional (2D) covalent bonding network in the ferroelectric phase at RT. The 2D interfaces of the ceramic particles in the aerosol, which are chemically activated by collision with the substrate, are most likely to recombine during the AD process and form a dense and hard coating at RT. When the PT film was heated to the paraelectric phase with a cubic structure at 1000 K, the c-axis orientation observed in the as-deposited state disappeared upon cooling owing to the degree of freedom in selecting the polar axis associated with the cubic-tetragonal phase transition. The spontaneous lattice distortion of the AD film at RT was smaller than that of the bulk ceramic. However, the phase transition temperature (TC) was approximately 100 K higher only during the first heating process from the c-oriented as-deposited state. The AD method stabilizes the ferroelectric phase up to a higher temperature, although the spontaneous polarization is suppressed. The ferroelectric ceramic materials synthesized mainly via particle collisions are assumed to have low ferroelectricity but high TC.

Enhanced ferroelectric, piezoelectric, and dielectric properties in potassium sodium niobate lead-free piezoelectric ceramics by constructing of multiple phase boundaries

In this work, we report excellent ferroelectric, piezoelectric, and dielectric properties in 0.95(K0.5−x/2Na0.5−x/2Lix)NbO3-0.04BaZrO3-0.01BiFeO3, ((KN)1−xLxN-BZ-BF) lead-free piezoelectric ceramics system by constructing a coexistence of orthorhombic-tetragonal phase boundary near the room temperature, and increasing of grain size as well through the conventional sintering process. The addition of Li+ ions, in ((KN)1−xLxN-BZ-BF) composition, increased the grain size, and shifted the orthorhombic-tetragonal phase transition to near room temperature, and shifted the tetragonal-cubic phase transition toward higher temperatures, since both large electrical properties, and moderate Curie temperature was achieved simultaneously. The sample with x = 0.03 showed large average grain size of (9 µm), and possess coexisting of orthorhombic-tetragonal phases, since the sample showed excellent physical properties, such as d33 = 330 pC/N, kp = 0.5, Pr = 25 µC/cm2, and εr = 1740. The origin of these decent combinations of physical properties owes to the coexistence of orthorhombic-tetragonal phase transition to near room temperature, large grain size, and hybridization between Bi 6p, and O 2p as well.

Sub-THz Raman response and soft phonon in tetragonal BaTiO3

ABSTRACT The sub-THz polarized Raman response was measured along the [100] direction in the tetragonal BaTiO3 single crystal. The soft phonon was detected as a resonance peak for the first time by Raman spectroscopy, thanks to the use of notch filters that suppressed elastic scattering. No relaxation peak was observed and we demonstrated that, contrary to the situation along the ferroelectric c-axis, the soft phonon is sufficient to describe the dielectric response of BaTiO3 along the a-axis, except in close vicinity of the tetragonal-cubic phase transition.

Controllable Piezo-flexoelectric Effect in Ferroelectric Ba0.7Sr0.3TiO3 Materials for Harvesting Vibration Energy.

The rapid development of the automotive and aerospace industries has led to an increasingly urgent need for electromechanical coupling materials and devices. Here, we have demonstrated the tunable piezo-flexoelectric effect in ferroelectric Ba0.7Sr0.3TiO3 materials for scavenging vibration energy. The positive peak output current of an ITO/Ba0.7Sr0.3TiO3/Ag cantilever device based on the flexoelectric effect is only 45 nA at room temperature, which is promoted to 90 nA by the piezo-flexoelectric effect. In addition, the piezo-flexoelectric current of the device can be further boosted to 270 nA by increasing the working temperature to 41.0 °C with a corresponding enhancement ratio of 348.28%. The significantly improved piezo-flexoelectric current is ascribed to the ultrahigh dielectric constant, which is related to the tetragonal-cubic phase transition of the Ba0.7Sr0.3TiO3 materials. This work reveals the temperature-modulated piezo-flexoelectric effect in ferroelectric Ba0.7Sr0.3TiO3 materials, providing a convenient route for scavenging and sensing of vibration energy.

Pressure induced superconductivity in nonlinear optical crystal ZnGeP2 and its capture at ambient pressure

An insulator could be driven into an emergent superconducting state when it is put in the extreme high-pressure condition, however, it is challenging to retain the obtained superconductivity to ambient pressure. Here we report on the creation and preservation of the superconductivity in chalcopyrite ZnGeP2, a commercial nonlinear optical crystal. Electrical transport measurements reveal that the superconductivity, induced at a critical pressure ∼17 GPa, is robust not only upon further compression to 52.9 GPa, but also in the process when the pressure is gradually released to ambient pressure. Synchrotron X-ray diffraction and Raman measurements document a structural origin of the unusual evolution of the superconductivity. The insulator-superconductor transition around 17 GPa is correlated with a structural transition from tetragonal to cubic phase, while the preserved superconductivity is consisitent with the irreversibility of the tetragonal-cubic phase transition and the presence of amorphization in the depressurized sample. These findings demonstrate a strategy to capture the high-pressure phase with emerging physical properties and will shed light on exploring novel superconductors by means of pressure treatment.

Cubic halide perovskites as potential low thermal conductivity materials: A combined approach of machine learning and first-principles calculations

Thermal conductivity is the key factor affecting thermoelectric properties of materials. Here, machine-learning techniques combined with first-principles calculations are used to identify the cubic halide perovskites ${\text{Cs}B\text{Br}}_{3}$ (B = Ca, Cd, and Sn) with ultralow thermal conductivity. Based on the Boltzmann transport equation within the relaxation time approximation, we demonstrate this type of perovskites have remarkably low lattice thermal conductivities ${\ensuremath{\kappa}}_{L}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}0.4\ensuremath{-}1$ W/mK at 300 K. We employ the self-consistent phonon theory incorporating both cubic and quartic anharmonicity, which is considered from the bubble and loop self-energy diagrams rather than many-body perturbation theory. We show that the approach yields a cubic-tetragonal phase transition of ${\mathrm{CsCaBr}}_{3}$ at temperature ${T}_{c}=226\ensuremath{-}265\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, in good agreement with the experimental value of 239 K. An anomalously temperature dependence of ${\ensuremath{\kappa}}_{L}$ is observed in ${\mathrm{CsCdBr}}_{3}$, where the coherent term account for 26% of the total lattice thermal conductivity. We also demonstrate that the hardening of vibrations in low-lying phonon modes offset the phonon population effect as temperature increases by reducing the available phase space.

Homogenous Sn-doped K(Ta,Nb)O3 single crystals and its high piezoelectric response

Perovskite K(Ta,Nb)O3 (KTN) single crystal has drawn great interests for its outstanding electro-optic performance and excellent piezoelectric response. However, growth of compositionally uniform KTN single crystals has always been a great challenge for the great segregation difference between Nb and Ta. In this work, we propose a thermal field optimization strategy to resolve this challenge. Homogenous Sn doped KTN (Sn:KTN) single crystal with significantly reduced composition gradient (0.003 mol/mm, 1/4–1/8 of other KTN system), minimal TC variation (13 °C) and excellent piezoelectric and dielectric response (d33 = 373 pC/N and ε33T = 5206) has been successfully achieved. We found that the functional properties of Sn:KTN were greatly affected by the near-room temperature tetragonal-cubic phase transition. From the intrinsic aspect, longitudinal lattice deformation becomes much easier, resulting in maximum piezoelectric (d33∗), dielectric (ε33T∗), elastic (s33E∗)and electromechanical coupling (k33∗) coefficients along polar direction [001]C. From the extrinsic aspect, both domain wall density and domain wall mobility are greatly improved for the reduced lattice distortion, which also contribute a lot to the functional properties. This work provides a simple and practical route for designing and growing high quality crystals, and more importantly, reveals the fundamental mechanism of the phase transitions/boundaries on the functional properties.

Tunning the linkage of structure units to enable stable spinel-based cathode in the wide potential window

The low practical capacity (<140 mA h g −1 ) of spinel phase LiMn 2−x Ni x O 4 (0 < x < 1) excludes it from high energy density lithium ion batteries (LIBs) for powering electric vehicles. Extending the operating potential window from 3.0 to 4.8 V to 2.0–4.8 V can double the capacity, but result in fast capacity decay due to the drastic cubic-tetragonal phase transition below 3.0 V, induced by serious Jahn-Teller (J-T) distortion of structure units (Mn 3+ )O 6 octahedra. Herein, we propose a novel strategy to suppress J-T distortion by tuning the linkage of structure units MnO 6 octahedra. The original full-vertex-sharing LiO 4 tetrahedra around MnO 6 octahedra in spinel phase are partially replaced by edge-sharing LiO 6 octahedra, which relate Mn-O bonds along the dz 2 direction with those along the d xy plane, thus significantly mitigating J-T distortion and suppressing the phase transition when discharged to 2.0 V. Following this strategy, the prepared spinel-based cathode achieves a high reversible capacity of about 290 mA h g −1 and an energy density up to 957 W h kg −1 with improved cycling stability. This work finds a new opportunity for the traditional spinel cathode towards applications in high energy density LIBs in a low cost and sustainable manner. Tunning the linkage of structure units MnO 6 octahedra in spinel-based cathode from full-vertex-sharing mode to partial-edge-sharing mode, greatly restricts Jahn-Teller distortion when discharged below 3 V and delivers a high capacity with decent cycling stability, which makes spinel cathode potential to power electric vehicles. • The structural origin for the Jahn-Teller effect in spinel phase was revealed based on the linkage way of MnO 6 octahedra. • Through a spinel/layered complex strategy, the local structure linkage of MnO 6 octahedra was partially converted to edge-sharing. • The change in the local structure linkage greatly suppressed the Jahn-Teller distortion and improved the cycling stability.

Effects of frequency, temperature, and dc bias electric field on the dielectric properties of methylammonium lead iodide from the perspective of a relaxor-like ferroelectric

This work reports dielectric properties of methylammonium lead iodide as a function of frequency, temperature, and dc bias electric field studied in terms of grain and grain boundary contributions. These results were analyzed from the perspective of a relaxor-like ferroelectric nature. The temperature dependence of dielectric permittivity at different frequencies showed a wide dispersion in the vicinity of the tetragonal-cubic phase transition, suggesting a relaxor ferroelectric feature after excluding artifacts as Maxwell-Wagner effects and dc contributions. The results from plots of the Vogel-Fulcher and the modified Curie-Weiss laws indicate the existence of a freezing temperature of the dipoles responsible for the relaxor character of the material (Tf ∼ 270 K) and a diffuse phase transition at high temperatures (γ ∼ 1.52-1.74). Measurements on heating and cooling protocols under a dc bias electric field suggest field-induced polarized domains like relaxor ferroelectric. The observed signatures of transitions between non-ergodic and ergodic relaxors to ferroelectric states were attributed to polar nano regions dynamics, suggesting a critical electric field around 0.03 kV cm−1.

On the question of the critical end point in the (1 − x)PbMg1/3Nb2/3O3-xPbTiO3 relaxor ferroelectric single crystals based on acoustic emission data studies

()PbMgNbOPbTiO3, [100]-oriented relaxor ferroelectric crystals were studied by means of dielectric and acoustic emission methods in a temperature range up to 200 °C and under a dc bias electric field up to 1.2 kV/cm. The monoclinic-tetragonal and tetragonal-cubic ferroelectric phase transitions were successfully detected by both experimental methods. The bias electric field dependence of the temperature of a dielectric constant maximum, related to tetragonal-cubic phase transitions, exhibits a V-shape effect with a threshold field of 0.5 kV/cm. In contrast, the bias electric field dependence of the temperature of a monoclinic-tetragonal phase transition exhibits no V-shape effect but points out a tricritical point at 0.7 kV/cm. These results have not confirmed the existence of the critical end point in the crystals under studies.

Terahertz Analysis of CH3NH3PbI3 Perovskites Associated with Graphene and Silver Nanowire Electrodes.

In order to investigate the thermal and chemical (in)stabilities of MAPbI3 incorporated with graphene and silver nanowire (AgNW) electrodes, we employed the terahertz (THz) time-domain spectroscopy, which has a unique ability to deliver the information of electrical properties and the intermolecular bonding and crystalline nature of materials. In in situ THz spectroscopy of MAPbI3, we observed a slight blue-shift in frequency of the 2 THz phonon mode as temperatures increase across the tetragonal-cubic structural phase transition. For MAPbI3 with the graphene top electrode, no noticeable frequency shift is observed until the temperature reaches the maximum operating temperature of solar cells (85 °C). Phonon frequency shift is sensitive to the strain-induced tilt of PbI6 octahedra and our results indicate that graphene forms a stable interface with MAPbI3 and is also effective in suppression of the undesirable phase transition. Meanwhile, for MAPbI3 coupled with the AgNW bottom electrode, the THz conductivity was found to be as low as that of the MAPbI3 single layer, attributed to the chemical reaction between Ag atoms and iodide ions. The THz conductivity is greatly increased when an ultrathin Al2O3 interlayer is introduced to cover the AgNW network via the atomic layer deposition (ALD) method. ALD of Al2O3 on the AgNW surfaces at low temperature guarantees a conformal coating, which strongly affects the ohmic contacts between the NWs. Our results demonstrate the advantage of THz spectroscopy for the comprehensive analysis of thermal and chemical stabilities of perovskites associated with the electrode materials.

Pressure-induced bandgap engineering of lead-free halide double perovskite (NH4)2SnBr6.

Lead-free halide double perovskites (HDPs) have recently been proposed as potential stable and environment-friendly alternatives to lead-based halide perovskites. Bandgap engineering plays a vital role in the optoelectronic applications of HDP materials. In this study, methods combining high-pressure techniques with density functional theory calculations were employed to implement the bandgap engineering of a classic HDP-based (NH4)2SnBr6. Under high pressure, (NH4)2SnBr6 exhibits a redshift of the bandgap with increasing pressure up to 6.3 GPa and a sudden blueshift up to 20.2 GPa, followed by a redshift at higher pressures, which is relevant to the cubic-tetragonal phase transition, direct-indirect transition, and amorphization, respectively. Our results enrich the understanding of the structural-optical properties of (NH4)2SnBr6 and reveal the special role of NH4+ cations in pressure-induced bandgap engineering, thus providing important information for application in optoelectronic devices and helping to design ideal materials with higher efficiency.

Facile Synthesis of Metastable Mg-Mn Spinel Nanoparticles for Magnesium-Ion Batteries

High energy and high density rechargeable batteries are indispensable for widespread portable electronic devices and the spreading of electric vehicles. Rechargeable Mg-ion batteries have gained much attention as promising alternatives to Li-ion batteries due to the high natural abundance, high volumetric energy density, and no dendrite formation of the Mg metal anode. While Mg-ion batteries are nowadays progressing rapidly, their practical use is still hampered by problems related to both cathode materials and electrolytes. One of the crucial problems is the very low rate capability at the cathodes due to the slow diffusion of Mg2+ ions in solids, therefore Mg-ion batteries work only at low current densities or at high temperature. Among various oxide cathode candidates for Mg-ion batteries, spinel oxides have a high redox potential and a relatively high ion-diffusivity. Especially in the common electrochemical window of electrolytes, MgMn2O4 spinel can exhibit a high theoretical capacity of 540 mAh g–1 using both Mn3+/Mn4+ and Mn3+/Mn2+ redox reactions (λ-MnO2 formation and rock-salt Mg2Mn2O4 formation reactions, respectively). However, these reactions should exhibit large polarizations because of the less reversible tetragonal-cubic phase transitions; MgMn2O4 has a tetragonal spinel structure due to the Jahn–Teller effect of Mn3+ ions, while λ-MnO2 and Mg2Mn2O4 are cubic phase. A suppression of the lattice distortion of MgMn2O4 is necessary for improving its cathode performance. Although a cubic MgMn2O4 phase is known as a metastable phase, this phase is only obtained at high-temperature or high-pressure in bulk. In this study, we aimed at synthesizing metastable cubic MgMn2O4 spinel nanoparticles under ambient conditions and applying them as cathodes for Magnesium-ion batteries.Mg-Mn spinel nanoparticles were synthesized by alcohol-reduction method[1] and hot-injection method. For the alcohol-reduction method, n-Bu4NMnO4 was reduced in anhydrous MgCl2 ethanol solution at room temperature to form fine particles. For the hot-injection method, MgCl2 and Mn(acac)3 were dissolved in anhydrous xylene solution with stearic acid and oleylamine, followed by water injection at 80ºC to form monodisperse nanoparticles. Charge/discharge tests between –1.0~1.0 V vs. Ag+/Ag was carried out using a three-electrode cell with an activated carbon anode and a 0.5 M Mg(ClO4)2/CH3CN electrolyte.The Rietveld refinements of XRD patterns suggested that the obtained samples were assigned to cubic phase Mg1–x Mn2O4, and a transformation to tetragonal phase was observed after annealing at 600ºC. Since the both methods utilize rapid nucleation reactions occurred at moderate temperature, once a meta-stable phase formed, the phase transition to more stable tetragonal phase hardly proceed. The cubic phase is obtained probably due to the rapid nucleation of cubic MgMn2O4 spinel nanoparticles at moderate temperature. According to TEM images, the primary particle size of both samples was below 10 nm, suggesting the rapid nucleation without crystal growth. However, the particles were aggregated to form large secondary particles. The electrochemical performances strongly depended on the morphology of secondary particles, therefore suppression of the aggregation is important for improving the performance. Reference: [1] H. Kobayashi, K. Yamaguchi, I. Honma, RSC Adv. 9 (2019) 36434–36439.

Evolution of ferroelastic domain walls during phase transitions in barium titanate nanoparticles

In this work, ferroelastic domain walls inside ${\mathrm{BaTiO}}_{3}$ (BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent x-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature.

Test of the Dynamic-Domain and Critical Scattering Hypotheses in Cubic Methylammonium Lead Triiodide

We investigate the hypothesis of dynamic tetragonal domains occuring in cubic CH$_3$NH$_3$PbI$_3$ using high-resolution neutron spectroscopy to study a fully deuterated single crystal. The $\textit{R}$-point scattering above the 327.5 K tetragonal-cubic phase transition is always resolution-limited in energy and therefore inconsistent with dynamic domain predictions. This behavior is instead consistent with the central peak phenomenon observed in other perovskites. The scattering may originate from small, static, tetragonal-phase domains nucleating about crystal defects, and the temperature dependence demonstrates the transition is first-order.

Intrinsic piezoelectricity in (K,Na)NbO3-based lead-free single crystal: Piezoelectric anisotropy and its evolution with temperature

Searching for lead-free piezoelectric materials with a large piezoelectricity and excellent thermal stability has been a major concern in both scientific research and practical applications. To understand the mechanism of high piezoelectricity and its temperature-dependent behavior in lead-free materials, we focus here on the tetragonal (K,Na,Li)(Nb,Ta)O3 single crystal and investigate the intrinsic d33* along arbitrary directions as well as its evolution with temperature. The synergistic influence of several factors (narrow tetragonal temperature interval, sharp tetragonal-cubic phase transition, and large PS below TC) leads to a strong anisotropy in the d33* profile, while a high d33* is obtained around θ = 45° over a wide temperature range. This work comprehensively reveals the physical mechanism of piezoelectric anisotropy in lead-free materials, which provides vital information to design high-performance lead-free piezoelectric materials through orientation engineering and lattice manipulation, which is expected to benefit a wide range of piezoelectric materials.

Phase relationships in the system ZnS-CuInS2: Insights from a nanoscale study of indium-bearing sphalerite

Abstract Micrometer- to submicrometer-scale indium-rich domains are preserved within sphalerite included in hornfels-hosted pyrrhotite from the Dulong polymetallic skarn, Yunnan, China. The nano-mineralogy of the ZnS-bearing blebs was investigated using scanning transmission electron microscopy on thinned foils extracted in situ from pyrrhotite. Indium incorporation in sphalerite occurs via the coupled substitution 2Zn2+ ↔ Cu+ + In3+; the results thus allow insights into phase relationships in the system ZnS-CuInS2 in which solubility limits are debated with respect to a cubic to tetragonal phase transition. The highest concentrations of In are measured in basket-weave domains from the smallest ZnS blebs or from un-patterned areas in coarser, irregular ZnS inclusions in pyrrhotite. Indium-rich domains contain 17–49 mol% CuInS2, whereas In-poor sphalerite contains &amp;lt;5 mol% CuInS2. Atomic-scale metal ordering observed in In-(Cu)-rich ZnS domains was modeled as mixed sites in a cubic structure with P43m symmetry and empirical formula [(Cu,In,Zn)3(Zn0.5Fe0.5)]4S4. This sphalerite modification is distinct from the cubic-tetragonal phase transition reported elsewhere for analogous, synthetic phases with abundant planar defects. The Zn1.5Fe0.5CuInS4 nanophase described here potentially represents a Fe-bearing polymorph of Zn2CuInS4, considered as an end-member in the sakuraiite solid-solution series. At ≤50 mol% CuInS2 in the ZnS-CuInS2 system, incorporation of In via coupled In+Cu substitution is promoted within a cubic ZnS phase with lower symmetry than sphalerite rather than into the spatially coexisting chalcopyrite of tetragonal symmetry. Solid-state diffusion accounts for phase re-equilibration resulting in the basket-weave textures typical of In-(Cu)-rich domains in the smallest blebs, whereas fluid percolation assists grain coarsening in the irregular inclusions. The results show evidence for the existence of a more complex phase transition than previously recognized from experimental studies, and, intriguingly, also to a potential eutectic in the system ZnS-CuInS2. Pyrrhotite-bearing hornfels in skarns may concentrate In and other critical metals hosted in sphalerite and related sulfides due to the efficient scavenging from fluid by these minerals and the subsequent preservation of those included phases by sealing within the pyrrhotite matrix.