Anion Disorder Research Articles

Covalent Stabilization of the Iridium-Containing Oxyhydrides Sr2Mn0.5Ir0.5O3.25H0.75 and Sr2Mn0.5Ir0.5O2.66H1.33.

Reaction between Sr2Mn0.5Ir0.5O4 and CaH2 or LiH yields the iridium-containing oxyhydride phases Sr2Mn0.5Ir0.5O3.25H0.75 or Sr2Mn0.5Ir0.5O2.66H1.33, respectively. Analysis of Mn K-edge XANES data indicate the presence of Ir3+ centers in these oxyhydride phases, whose low-spin d6 configuration is consistent with the "covalent stabilization" of the metastable oxyhydride phases, as seen previously in analogous ruthenium and rhodium containing materials. Neutron powder diffraction data indicate the hydride ions are located exclusively within the "equatorial" anion sites of Sr2Mn0.5Ir0.5O3.25H0.75. In contrast, hydride ions are observed on both the equatorial and axial anion sites of Sr2Mn0.5Ir0.5O2.66H1.33. This highly unusual anion distribution is attributed to a combination of the strong trans-influence of Ir-H σ-bonds and the stabilization of fac-IrO3H3 centers by spin-orbit coupling effects. Magnetization data indicate that Sr2Mn0.5Ir0.5O4 and Sr2Mn0.5Ir0.5O3.25H0.75 adopt spin glass states at low temperature, behavior which is attributable to the cation disorder in Sr2Mn0.5Ir0.5O4 and the cation and anion disorder in Sr2Mn0.5Ir0.5O3.25H0.75. In contrast, magnetization data collected from Sr2Mn0.5Ir0.5O2.66H1.33 show no evidence of any magnetic phase transition down to 5 K, consistent with the dilution of the magnetic network by the introduction of diamagnetic Ir3+ on the formation of the oxyhydride phase.

Site Disorder Drives Cyanide Dynamics and Fast Ion Transport in Li6PS5CN.

Halide argyrodite solid-state electrolytes of the general formula Li6PS5 X exhibit complex static and dynamic disorder that plays a crucial role in ion transport processes. Here, we unravel the rich interplay between site disorder and dynamics in the plastic crystal argyrodite Li6PS5CN and the impact on ion diffusion processes through a suite of experimental and computational methodologies, including temperature-dependent synchrotron powder X-ray diffraction, AC electrochemical impedance spectroscopy, 7Li solid-state NMR, and machine learning-assisted molecular dynamics simulations. Sulfide and (pseudo)halide site disorder between the two anion sublattices unilaterally improves long-range lithium diffusion irrespective of the (pseudo)halide identity, which demonstrates the importance of site disorder in dictating bulk ionic conductivity in the argyrodite family. Furthermore, we find that anion site disorder modulates the presence and time scales of cyanide rotational dynamics. Ordered configurations of anions enable fast, quasi-free rotations of cyanides that occur on time scales of 1011 Hz at T = 300 K. In contrast, we find that cyanide dynamics are slow or frozen in Li6PS5CN when site disorder between the cyanide and sulfide sublattices is present at T = 300 K. We rationalize the observed differences in cyanide dynamics in the context of elastic dipole interactions between neighboring cyanide anions and local strain induced by the configurations of site disorder that may impact the energetic landscape for cyanide rotational dynamics. Through this study, we find that anion disorder plays a decisive role in dictating the extent and time scales of both lithium ion and cyanide dynamics in Li6PS5CN.

Molecular O2 Dimers and Lattice Instability in a Perovskite Electrocatalyst.

Structural degradation of oxide electrodes during the electrocatalytic oxygen evolution reaction (OER) is a major challenge in water electrolysis. Although the OER is known to induce changes in the surface layer, little is known about its effect on the bulk of the electrocatalyst and its overall phase stability. Here, we show that under OER conditions, a highly active SrCoO3-x electrocatalyst develops bulk lattice instability, which results in the formation of molecular O2 dimers inside the bulk and nanoscale amorphization induced via chemo-mechanical coupling. Using high-resolution resonant inelastic X-ray scattering and first-principles calculations, we unveil the potential-dependent evolution of lattice oxygen inside the perovskite and demonstrate that O2 dimers are stable in a densely packed crystal lattice, thus challenging the assumption that O2 dimers require sufficient interatomic spacing. We also show that the energy cost of local atomic rearrangements in SrCoO3-x becomes very low under the OER conditions, leading to an unusual amorphization under intercalation-induced stress. As a result, we propose that the amorphization energy can be calculated from the first principles and can be used to assess the stability of electrocatalysts. Our study demonstrates that extreme oxidation of electrocatalysts under OER can intrinsically destabilize the lattice and result in bulk anion redox and disorder, suggesting why some oxide materials are unstable and develop a thick amorphous layer under water electrolysis conditions.

Correlated Anion Disorder in Heteroanionic Cubic TiOF2.

Resolving anion configurations in heteroanionic materials is crucial for understanding and controlling their properties. For anion-disordered oxyfluorides, conventional Bragg diffraction cannot fully resolve the anionic structure, necessitating alternative structure determination methods. We have investigated the anionic structure of anion-disordered cubic (ReO3-type) TiOF2 using X-ray pair distribution function (PDF), 19F MAS NMR analysis, density functional theory (DFT), cluster expansion modeling, and genetic-algorithm structure prediction. Our computational data predict short-range anion ordering in TiOF2, characterized by predominant cis-[O2F4] titanium coordination, resulting in correlated anion disorder at longer ranges. To validate our predictions, we generated partially disordered supercells using genetic-algorithm structure prediction and computed simulated X-ray PDF data and 19F MAS NMR spectra, which we compared directly to experimental data. To construct our simulated 19F NMR spectra, we derived new transformation functions for mapping calculated magnetic shieldings to predicted magnetic chemical shifts in titanium (oxy)fluorides, obtained by fitting DFT-calculated magnetic shieldings to previously published experimental chemical shift data for TiF4. We find good agreement between our simulated and experimental data, which supports our computationally predicted structural model and demonstrates the effectiveness of complementary experimental and computational techniques in resolving anionic structure in anion-disordered oxyfluorides. From additional DFT calculations, we predict that increasing anion disorder makes lithium intercalation more favorable by, on average, up to 2 eV, highlighting the significant effect of variations in short-range order on the intercalation properties of anion-disordered materials.

Magnetic Bistability in the Crystaline Fullerene Radical Salt

Comprehensive SummaryCompounds with magnetic bistability is highly attractive for the construction of switches, thermal sensors, information‐storage media and memory devices. Herein, we report a crystalline fullerene radical anion salt [Na(THF)5]C60 (1), obtained by the reduction of C60 with Na in solution, which exhibits magnetic bistability accompanied with one magnetostructural transition, giving low temperature (LT), high temperature (HT) phases and a metastable phase. Fullerene radical anions (C60•‐) in the structure were found arranged into a three‐dimensional close‐packed honeycomb subnetwork with hollow channels occupied by complex cations [Na(THF)5]+ as spacers. The magnetic bistability is attributed to structural transitions as the LT−IT phase transition was associated with the orientational disorder of fullerene anions and the distortion of the complex cations [Na(THF)5]+ in the 3D packing of fullerene anions.

Acoustic Shock Wave-Induced Amorphous to Crystalline Phase Transitions of Li2SO4─Raman Spectroscopic and Thermal Calorimetric Approach.

In this context, we have reexamined the acoustical shock wave-induced amorphous-glassy-crystalline-amorphous phase transitions in the Li2SO4 sample under 0, 1, 2, and 3 shocked conditions by implementing the detailed Raman spectroscopic approach. The recorded Raman spectroscopic data clearly reveal that the transition from the amorphous-glassy-crystalline state occurs because of a significant reduction of the translational disorder of lithium cations, particularly [Li (2)] ions wherein a slight reduction of the librational disorder of SO4 anions takes place, whereas the crystalline to amorphous transition occurs only at the third shocked condition because of the librational disorder of SO4 anions. The double degenerate υ2 and υ4 Raman modes provide a clear indication of the occurrence of the librational disorder of SO4 anions at the third shocked condition. Followed by the internal Raman modes, a detailed discussion is provided on the external Raman modes of the Li ions and SO4 ions with respect to the observed phase transitions, wherein it is found that the regions of lattice modes are significantly altered at each and every point of phase transition. Furthermore, the thermal and magnetic measurements have been performed for the above-mentioned state of Li2SO4 samples, whereby the obtained results of the magnetic loops and the thermal property resemble the observed structural transitions with respect to the number of shock pulses such that the inter-relationship of the structure-electrical-magnetic-thermal properties of Li2SO4 could be explored.

Synthesis, Characterization, and Polymorphism of [H3O][NbF6]: A Polar and Possibly Ferroelectric Oxonium Salt

Abstract[H3O][NbF6] was obtained from the controlled hydrolysis of NbF5 in anhydrous liquid HF. It adopts a polar, orthorhombic crystal structure with space group Iba2 (no. 45, oI88) at room temperature. A first‐order phase transition at 137 K leads to a cubic non‐centrosymmetric polymorph in space group I213 (no. 199, cI88). This low‐temperature modification results from a distinct rotation of the [H3O]+ cations canceling their polar orientation in the room temperature phase. Quantum‐chemical calculations estimate a rotational barrier between 5.8 to 6.4 kJ/mol. At a temperature of 363 K, the compound adopts a centrosymmetric, cubic crystal structure in space group Pm m (no. 221, cP11) that shows rotational disorder of cations and anions. The transition from the polar phase at room temperature to the centrosymmetric phase at high temperature not only reveals the plastic nature of the high‐temperature structure but also hints at potential ferroelectric properties, underscoring the multifaceted behavior of [H3O][NbF6] across different temperature regimes.

The layered RuBr3–RuI3 honeycomb system

A honeycomb layered Ru(Br1−xIx)3 solid solution is prepared through a high-pressure synthesis method, with anion disorder and strong spin–orbit coupling. Their electronic and magnetic properties vary dramatically with changing chemical composition.

Understanding the anion disorder governing lithium distribution and diffusion in an argyrodite Li6PS5Cl solid electrolyte

S/Cl anion disorder greatly enhances the stability and conductivity of Li6PS5Cl by inducing Li redistribution that enriches and depletes Li ions around S and Cl, respectively.

Signature of Topological semimetal in harmonic-honeycomb ReO3

Transition-metal honeycomb compounds are capturing scientific attention due to their distinctive electronic configurations, underscored by the triangular-lattice spin-orbit coupling and competition between multiple interactions, paving the way for potential manifestations of phenomena such as Dirac semimetal, superconductivity, and quantum spin liquid states. These compounds can undergo discernible pressure-induced alterations in their crystallographic and electronic paradigms, as exemplified by our high-pressure (HP) synthesis and exploration of the honeycomb polymorph of ReO3 (P6322). This HP-P6322 polymorph bears a phase transition from P6322 to P63/mmc upon cooling around Tp = 250 K, as evidenced by the evolution of temperature-dependent magnetization (M-T curves), cell dimension and conductivity initiated by an inherent bifurcation of the oxygen position in the ab plane. Insightful analysis of its band structure positions suggests this HP-P6322 polymorph being a plausible candidate for Dirac semimetal properties. This phase transition evokes anomalies in the temperature-dependent variation of paramagnetism (non-linearity) and a crossover from semiconductor to temperature-independent metal, showing a temperature independent conductivity behavior below ∼200 K. Under increasing external pressure, both the Tp and resistance of this HP-polymorph are slightly magnetic-field dependent and undergo a “V”-style evolution (decreasing and then increasing) before becoming pressure independent up to 20.2 GPa. Theoretical calculations pinpoint this anionic disorder as a probable catalyst for the decrement in the conductive efficiency and muted temperature-dependent conductivity response.

Structural ordering studies of Nd3+ ion in eulytite-type M3Y(PO4)3 (M=Sr2+ or Ba2+) phosphates. First translucent ceramics from micro-crystalline cubic powders

Cubic phosphates of M3Y(PO4)3 (M = Sr2+ or Ba2+) from the eulytite family were studied as potential candidates for new optical polycrystalline ceramics showing transparency. Two series of Nd3+-doped Ba3Y(PO4)3 and Sr3Y(PO4)3 micro-powders were synthesized via solid-state reaction method. The correlation between their structure and spectroscopic properties and the effect of divalent Sr2+/Ba2+ cation creating a network was analyzed in detail. The crystal structure of Ba3Y(PO4)3 and Sr3Y(PO4)3 was solved ab initio based on their single crystals. Y3+ and Sr2+/Ba2+ ions do not share the same symmetry site and some slight distortion over crystallographic positions occupied by divalent and trivalent ions in the cationic sublattice was visible. Nd3+ reside at the Y3+ site but partial substitution of Ba2+ or Sr2+ cannot be excluded. The cationic and anionic disorder is reflected in spectroscopic properties. Nd3+ ion used as a structural probe shows multisite character and inhomogeneous distribution in both types of material, as determined using absorption spectroscopy at 4.2 K and laser-excited site-selective luminescence spectroscopy at 77 K. This is also clearly seen in excited state dynamics of 4F3/2 doublet analysis. A greater order for barium host lattice was observed. Also, the first un-doped translucent ceramics were obtained from micro-crystalline powders.

Si-doped Li6PS5I with enhanced conductivity enables superior performance for all-solid-state lithium batteries

Lithium argyrodites Li6PS5X (X = Cl, Br, I) show great potential as solid electrolytes for solid-state lithium batteries due to their high Li-ion conductivities and excellent electrode compatibility. However, the relatively low conductivity of Li6PS5I (10−6 mS/cm) compared to the other two compositions limits its applications. Herein, Si-doped Li6.5P0.5Si0.5S5I electrolyte is designed and synthesized with superior high conductivity of 3.6 mS/cm. Structural characterization proves the increase due to the anion disorder and volume expansion caused by Si-doping. However, the poor interfacial stability between layered oxide cathode LiNi0.6Co0.2Mn0.2O2 and Li6.5P0.5Si0.5S5I inhibits its battery performance. By introducing Li3InCl6 electrolyte in the configuration, the corresponding battery delivers high initial discharge capacity of 150.2 mAh/g and superior cyclability during 250 cycles at 0.5 C. This work offers design strategy to obtain Li6PS5I-based electrolytes for high performance solid-state batteries.

Regulating the anion disorder and synthesizing the low-cost and high-performance Li6−xPS5−xCl1+x solid electrolytes

Sulfide solid electrolytes have attracted wide attention due to their high ionic conductivity. Li6PS5Cl is one of the most promising electrolytes for all-solid-state lithium batteries with high-energy-density. It is reported that the disordered arrangement of Cl- in the aragonite structure has a significant effect on the conductivity of Li6PS5Cl. In this work, the disorder of anion sites is modestly regulated by adjusting the ratio of S and Cl in Li6−xPS5−xCl1+x electrolyte. In addition, to verify the commonality of this strategy, Li5.6PS4.6Cl1.4 is prepared using self-made Li2S, and similar results are obtained. On this basis, the density of Li5.6PS4.6Cl1.4 electrolyte sheet is improved by hot-pressing, which can successfully inhibit the growth of lithium dendrites. As expected, the ionic conductivity of the sulfide electrolyte and the electrochemical performance of the ASSLBs are greatly improved. A highest ionic conductivity of 8.2 × 10−3 S cm−1 of Li5.6PS4.6Cl1.4 is obtained at room temperature. A high capacity retention of 99.99 % is maintained for NCM811 @Li2O//Li5.6PS4.6Cl1.4//Li-In system after 400 cycles at 1 C with ultra-high cathode loading of 35.6 mg cm−2. Besides, the cost of sulfide electrolytes can be reduced as the decreased usage of Li2S. This work proposes a novel synthesis method of low-cost and high-performance Li6−xPS5−xCl1+x electrolytes.

Configurational and Dynamical Heterogeneity in Superionic Li5.3PS4.3Cl1.7−xBrx

AbstractThe correlation between lattice chemistry and cation migration in high‐entropy Li+ conductors is not fully understood due to challenges in characterizing anion disorder. To address this issue, argyrodite family of Li+ conductors, which enables structural engineering of the anion lattice, is investigated. Specifically, new argyrodites, Li5.3PS4.3Cl1.7−xBrx (0 ≤ x ≤ 1.7), with varying anion entropy are synthesized and X‐ray diffraction, neutron scattering, and multinuclear high‐resolution solid‐state nuclear magnetic resonance (NMR) are used to determine the resulting structures. Ion and lattice dynamics are determined using variable‐temperature multinuclear NMR relaxometry and maximum entropy method analysis of neutron scattering, aided by constrained ab initio molecular dynamics calculations. 15 atomic configurations of anion arrangements are identified, producing a wide range of local lattice dynamics. High entropy in the lattice structure, composition, and dynamics stabilize otherwise metastable Li‐deficient structures and flatten the energy landscape for cation migration. This resulted in the highest room‐temperature ionic conductivity of 26 mS cm−1 and a low activation energy of 0.155 eV realized in Li5.3PS4.3Cl0.7Br, where anion disorder is maximized. This study sheds light on the complex structure–property relationships of high‐entropy superionic conductors, highlighting the significance of heterogeneity in lattice dynamics.

Impact of Anionic Ordering on the Iron Site Distribution and Valence States in Oxyfluoride Sr2FeO3+xF1-x (x = 0.08, 0.2) with a Layered Perovskite Network.

Sr2FeO3F, an oxyfluoride compound with an n = 1 Ruddlesden-Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO2 atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe4+ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr2FeO3.2F0.8 oxyfluoride, containing 20% of Fe4+, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr2FeO3.2F0.8 and its use in MIEC-based devices, such as solid oxide fuel cells.

Ba6Ge2Se12 and Ba7Ge2Se17: Two Centrosymmetric Barium Seleno-Germanates with Polyatomic Anion Disorder

Herein, the crystal structures and physical properties of two previously unreported barium seleno-germanates, Ba6Ge2Se12 and Ba7Ge2Se17, are presented. Ba6Ge2Se12 adopts the P21/c space group with a = 10.0903(2) Å, b = 9.3640(2) Å, c = 25.7643(5) Å, and β = 90.303(1)°, whereas Ba7Ge2Se17 crystallizes in the Pnma space group with a = 12.652(1) Å, b = 20.069(2) Å, c = 12.3067(9) Å. Both structures feature polyatomic anion disorder: [Se2]2- in the case of Ba6Ge2Se12 and [GeSe5]4- in the case of Ba7Ge2Se17. The anion disorder is verified by comparing pair distribution functions of ordered and disordered models of the structures. These anions are split unevenly across two possible sets of atomic coordinates. The optical band gaps obtained from the powdered samples are found to be 1.75 and 1.51 eV for Ba6Ge2Se12 and Ba7Ge2Se17, respectively. Differential scanning calorimetry experiments indicate that the compounds are stable under the exclusion of air up to at least 673 K. The thermal diffusivity measurements revealed thermal conductivities reaching values as low as 0.33 W m-1 K-1 in both compounds at 573 K.

The Heat Capacity of PuO2 at High Temperature: Molecular Dynamics Calculations

Abstract A new generation of fast breeder reactors (FBRs) is under development with the objective of making nuclear energy more sustainable. Most promising reactor designs are loaded, at least during their early phase of deployment, with UO2–PuO2 mixed oxide fuel (MOX). Concentrations of plutonium dioxide that are foreseen for FBRs range up to 30 mol%. This highlights the need for a sound and deep knowledge of the thermophysical properties of PuO2. This statement is valid in the case of heat capacity, as evaluations on MOX fuel are usually carried out by using the Neumann–Kopp rule. Heat capacity is relevant for thermal conductivity and performance under transient conditions. However, measurements on the heat capacity of plutonium dioxide are scarce or even lacking at high temperature. Numerical methodologies such as molecular dynamics (MD) calculations have been employed to overcome the difficulties encountered in experimental measurements. Besides numerical also theoretical models have been applied as valuable tools for interpretation of enthalpy measurements. Nevertheless, due to the mentioned lack of experimental measurement issues such as the existence of the Bredig transition and the formation of defects at high temperatures are still debated in nuclear fuel research. Excess enthalpy seen in measurements of actinides oxides has been explained by means of either electronic disorder or anion disorder. In the case of plutonium dioxide, a common consensus has been reached on the hypothesis that anion disorder leads to a significant increase in heat capacity at high temperature. Konings and Beneš have developed a model that accounts for this phenomenon. Their correlation has been often included in models of heat capacity and employed for recommendations. However, in the high-temperature region, MD calculations showed an underestimation of model predictions that was not compensated by the presence of a peak of heat capacity that has been interpreted as the Bredig transition. Based on these observations, this paper presents MD evaluations on the heat capacity of PuO2 at high temperature that are mostly focused on the formation energy of oxygen Frenkel pairs (OFPs) and its correlation with the model proposed by Konings and Beneš. Besides an interatomic potential published in the open literature and developed in compliance with the experimental thermal expansion of PuO2, a second interatomic potential has been applied in calculations. This latter is featured by a lower formation energy of OFP. The contribution due to defects formation was calculated by means of a simplified theoretical model of heat capacity. Results of calculations in the very high-temperature domain showed an increase in the contribution due to OFP defects consistent with the model by Konings and Beneš. Predictions suggest the onset of a premelting transition around 85% of melting temperature without the presence of a peak of heat capacity. Major deviations from the recommended model have been noted in the intermediate temperature region where the effect of clustering of defects should play a significant role. Therefore, the value of formation energy of OFP proposed by Konings and Beneš could be interpreted as an effective value that accounts for the two processes (defects clustering and premelting transition) that could contribute, according to our results, to the heat capacity of plutonium dioxide at high temperature. This conclusion is consistent with the numerical evaluations of OFP formation energy that are in general higher than proposed by Konings and Beneš.

Order–disorder transition and thermal conductivities of the (NdSmEuGd)(1-x)/2Dy2xZr2O7 series

Rare-earth zirconates with pyrochlore and fluorite structures have recently been identified as promising thermal barrier coating materials owing to their low thermal conductivities. In this study, six samples with the general formula (NdSmEuGd)(1-x)/2Dy2xZr2O7 were synthesized to further reduce the thermal conductivity. X-ray diffraction and Raman spectroscopy showed that the transition from an ordered pyrochlore to a disordered fluorite structure is due to cation and anion disorder. Transmission electron microscopy showed that anion disorder occurred before cation disorder. A modified mass disorder parameter was introduced into this system, which can describe the change in thermal conductivity well. This parameter can be a basis for designing more complex materials with lower thermal conductivities.

Effect of f–d and d–d Interactions on Dielectric and Optical Properties of Pyrochlore Eu2−xFexTi2O7

X‐ray diffraction, Raman spectroscopy, Fourier‐transform infrared (FTIR) spectroscopy, dielectric property, and UV–vis spectroscopy studies of pyrochlore Eu2−xFexTi2O7 have been performed. The X‐ray pattern shows that Eu2−xFexTi2O7 has a pure pyrochlore phase up to x = 0.2. The structural analysis shows a strong distortion of TiO6 octahedral due to the presence of the interstitial anionic vacant site. A significantly increased band intensity of FTIR spectrum with Fe doping suggests the enhanced anionic disorder in the system. The broadening of Raman peaks with increasing x supports the increased disorder in TiO6 octahedral. Dielectric study of Eu2Ti2O7 shows a diffused dielectric transition below 150 K. The dielectric diffusiveness and transition temperature get enhanced with Fe doping due to enhanced anionic distortion. The dielectric constant of Eu2−xFexTi2O7 is also increased on Fe doping. The UV–vis spectrum exhibits a strong absorbance in the UV region and a redshift with Fe doping. The Tauc plot of UV–vis spectrum shows multiple successive valence band edges and the bandgap gets reduced with Fe doping due to the formation of new interbands. Thus, these materials are interesting for people searching for material to be used in tunable electrical and optical devices.

Dynamic shock wave induced order-disorder type phase transition of sodium nitrate single crystal

In this letter, we demonstrate the dynamic shock wave induced order-disorder type phase transition exhibited by sodium nitrate single crystal and the results have been evaluated by diffraction, spectroscopic and microscopic techniques. The changeover of order into the disordered phase of NaNO3 crystal at shocked conditions is due to the occurrence of rotational disorder of planer anions (NO3).