Structural Polymorphism in “Kesterite” Cu₂ZnSnS₄: Raman Spectroscopy and First-Principles Calculations Analysis

This work presents a comprehensive analysis of the structural and vibrational properties of the kesterite Cu₂ZnSnS₄ (CZTS, <i>I</i>4̅ space group) as well as its polymorphs with the space groups <i>P</i>4̅2<i>c</i> and <i>P</i>4̅2<i>m</i>, from both experimental and theoretical point of views. Multiwavelength Raman scattering measurements performed on bulk CZTS polycrystalline samples were utilized to experimentally determine properties of the most intense Raman modes expected in these crystalline structures according to group theory analysis. The experimental results compare well with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Vibrational patterns of the most intense fully symmetric modes corresponding to the <i>P</i>4̅2<i>c</i> structure were compared with the corresponding modes in the <i>I</i>4̅ CZTS structure. The results point to the need to look beyond the standard phases (kesterite and stannite) of CZTS while exploring and explaining the electronic and vibrational properties of these materials, as well as the possibility of using Raman spectroscopy as an effective technique for detecting the presence of different crystallographic modifications within the same material

Order–Disorder Transitions and Superionic Conductivity in the Sodium <i>nido</i>-Undeca(carba)borates

The salt compounds NaB₁₁H₁₄, Na-7-CB₁₀H₁₃, Li-7-CB₁₀H₁₃, Na-7,8-C₂B₉H₁₂, and Na-7,9-C₂B₉H₁₂ all contain geometrically similar, monocharged, <i>nido</i>-undeca­(carba)­borate anions (i.e., truncated icosohedral-shaped clusters constructed of only 11 instead of 12 {B–H} + {C–H} vertices and an additional number of compensating bridging and/or terminal H atoms). We used first-principles calculations, X-ray powder diffraction, differential scanning calorimetry, neutron vibrational spectroscopy, neutron elastic-scattering fixed-window scans, quasielastic neutron scattering, and electrochemical impedance measurements to investigate their structures, bonding potentials, phase-transition behaviors, anion orientational mobilities, and ionic conductivities compared to those of their <i>closo</i>-poly­(carba)­borate cousins. All exhibited order–disorder phase transitions somewhere between room temperature and 375 K. All disordered phases appear to possess highly reorientationally mobile anions (> ∼10¹⁰ jumps s^–1 above 300 K) and cation-vacancy-rich, close-packed or body-center-cubic-packed structures [like previously investigated <i>closo</i>-poly­(carba)­borates]. Moreover, all disordered phases display superionic conductivities but with generally somewhat lower values compared to those for the related sodium and lithium salts with similar monocharged 1-CB₉H₁₀^– and CB₁₁H₁₂^– <i>closo</i>-carbaborate anions. This study significantly expands the known toolkit of solid-state, poly­(carba)­borate-based salts capable of superionic conductivities and provides valuable insights into the effect of crystal lattice, unit cell volume, number of carbon atoms incorporated into the anion, and charge polarization on ionic conductivity

Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH₃, where M = K, Rb, Cs, K_0.5Rb_0.5, K_0.5Cs_0.5, and Rb_0.5Cs_0.5) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH₃^– anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. This dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials

Structural and Dynamical Trends in Alkali-Metal Silanides Characterized by Neutron-Scattering Methods

Structural, vibrational, and dynamical properties of the mono- and mixed-alkali silanides (MSiH₃, where M = K, Rb, Cs, K_0.5Rb_0.5, K_0.5Cs_0.5, and Rb_0.5Cs_0.5) were investigated by various neutron experiments, including neutron powder diffraction (NPD), neutron vibrational spectroscopy (NVS), neutron-scattering fixed-window scans (FWSs), and quasielastic neutron scattering (QENS) measurements. Structural characterization showed that the mixed compounds exhibit disordered (α) and ordered (β) phases for temperatures above and below about 200–250 K, respectively, in agreement with their monoalkali correspondents. Vibrational and dynamical properties are strongly influenced by the cation environment; in particular, there is a red shift in the band energies of the librational and bending modes with increasing lattice size as a result of changes in the bond lengths and force constants. Additionally, slightly broader spectral features are observed in the case of the mixed compounds, indicating the presence of structural disorder caused by the random distribution of the alkali-metal cations within the lattice. FWS measurements upon heating showed that there is a large increase in reorientational mobility as the systems go through the order–disorder (β–α) phase transition, and measurements upon cooling of the α-phase revealed the known strong hysteresis for reversion back to the β-phase. Interestingly, at a given temperature, among the different alkali silanide compounds, the relative reorientational mobilities of the SiH₃^– anions in the α- and β-phases tended to decrease and increase, respectively, with increasing alkali-metal mass. This dynamical result might provide some insights concerning the enthalpy–entropy compensation effect previously observed for these potentially promising hydrogen storage materials

Comparison of Anion Reorientational Dynamics in MCB₉H₁₀ and M₂B₁₀H₁₀ (M = Li, Na) via Nuclear Magnetic Resonance and Quasielastic Neutron Scattering Studies

The disordered phases of the 1-carba-<i>closo</i>-decaborates LiCB₉H₁₀ and NaCB₉H₁₀ exhibit the best solid-state ionic conductivities to date among all known polycrystalline competitors, likely facilitated in part by the highly orientationally mobile CB₉H₁₀^– anions. We have undertaken both NMR and quasielastic neutron scattering (QENS) measurements to help characterize the monovalent anion reorientational mobilities and mechanisms associated with these two compounds and to compare their anion reorientational behaviors with those for the divalent B₁₀H₁₀^2– anions in the related Li₂B₁₀H₁₀ and Na₂B₁₀H₁₀ compounds. NMR data show that the transition from the low-<i>T</i> ordered to the high-<i>T</i> disordered phase for both LiCB₉H₁₀ and NaCB₉H₁₀ is accompanied by a nearly two-orders-of-magnitude increase in the reorientational jump rate of CB₉H₁₀^– anions. QENS measurements of the various disordered compounds indicate anion jump correlation frequencies on the order of 10¹⁰–10¹¹ s^–1 and confirm that NaCB₉H₁₀ displays jump frequencies about 60% to 120% higher than those for LiCB₉H₁₀ and Na₂B₁₀H₁₀ at comparable temperatures. The <i>Q</i>-dependent quasielastic scattering suggests similar small-angular-jump reorientational mechanisms for the different disordered anions, changing from more uniaxial in character at lower temperatures to more multidimensional at higher temperatures, although still falling short of full three-dimensional rotational diffusion below 500 K within the nanosecond neutron window

Stabilizing Superionic-Conducting Structures via Mixed-Anion Solid Solutions of Monocarba-<i>closo</i>-borate Salts

Solid lithium and sodium <i>closo</i>-polyborate-based salts are capable of superionic conductivities surpassing even liquid electrolytes, but often only at above-ambient temperatures where their entropically driven disordered phases become stabilized. Here we show by X-ray diffraction, quasielastic neutron scattering, differential scanning calorimetry, NMR, and AC impedance measurements that by introducing “geometric frustration” via the mixing of two different <i>closo</i>-polyborate anions, namely, 1-CB₉H₁₀^– and CB₁₁H₁₂^–, to form solid-solution anion-alloy salts of lithium or sodium, we can successfully suppress the formation of possible ordered phases in favor of disordered, fast-ion-conducting alloy phases over a broad temperature range from subambient to high temperatures. This result exemplifies an important advancement for further improving on the remarkable conductive properties generally displayed by this class of materials and represents a practical strategy for creating tailored, ambient-temperature, solid, superionic conductors for a variety of upcoming all-solid-state energy devices of the future

Scalable Heating-Up Synthesis of Monodisperse Cu₂ZnSnS₄ Nanocrystals

Monodisperse Cu₂ZnSnS₄ (CZTS) nanocrystals (NCs), with quasi-spherical shape, were prepared by a facile, high-yield, scalable, and high-concentration heat-up procedure. The key parameters to minimize the NC size distribution were efficient mixing and heat transfer in the reaction mixture through intensive argon bubbling and improved control of the heating ramp stability. Optimized synthetic conditions allowed the production of several grams of highly monodisperse CZTS NCs per batch, with up to 5 wt % concentration in a crude solution and a yield above 90%

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K

Nature of Decahydro-<i>closo</i>-decaborate Anion Reorientations in an Ordered Alkali-Metal Salt: Rb₂B₁₀H₁₀

The ordered monoclinic phase of the alkali-metal decahydro-<i>closo</i>-decaborate salt Rb₂B₁₀H₁₀ was found to be stable from about 250 K all the way up to an order–disorder phase transition temperature of ≈762 K. The broad temperature range for this phase allowed for a detailed quasielastic neutron scattering (QENS) and nuclear magnetic resonance (NMR) study of the protypical B₁₀H₁₀^2– anion reorientational dynamics. The QENS and NMR combined results are consistent with an anion reorientational mechanism comprised of two types of rotational jumps expected from the anion geometry and lattice structure, namely, more rapid 90° jumps around the anion <i>C</i>₄ symmetry axis (e.g., with correlation frequencies of ≈2.6 × 10¹⁰ s^–1 at 530 K) combined with order of magnitude slower orthogonal 180° reorientational flips (e.g., ≈3.1 × 10⁹ s^–1 at 530 K) resulting in an exchange of the apical H (and apical B) positions. Each latter flip requires a concomitant 45° twist around the <i>C</i>₄ symmetry axis to preserve the ordered Rb₂B₁₀H₁₀ monoclinic structural symmetry. This result is consistent with previous NMR data for ordered monoclinic Na₂B₁₀H₁₀, which also pointed to two types of anion reorientational motions. The QENS-derived reorientational activation energies are 197(2) and 288(3) meV for the <i>C</i>₄ fourfold jumps and apical exchanges, respectively, between 400 and 680 K. Below this temperature range, NMR (and QENS) both indicate a shift to significantly larger reorientational barriers, for example, 485(8) meV for the apical exchanges. Finally, subambient diffraction measurements identify a subtle change in the Rb₂B₁₀H₁₀ structure from monoclinic to triclinic symmetry as the temperature is decreased from around 250 to 210 K