Temperature-dependent X-ray Diffraction Research Articles

Formation and Stability of Nontoxic Perovskite Precursor.

Strontium, calcium, and magnesium silicate hydrate phases are synthesized by the reaction between silica and solution of metal hydroxides. The kinetics of the reaction is recorded using a quartz crystal microbalance (QCM), continuously monitoring the change in frequency and dissipation energy. Based on QCM results, it is shown that properties of solutions like the pH-value or the type of ions play a pivotal function on the rate-determining stage of the reaction, the thickness of the diffuse layer, the formation of carbonates, as well as the kinetics of the formed phases. Further properties of the reaction products are investigated using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and infrared spectroscopy (IR). With the help of thermogravimetric analysis (TGA) and temperature-dependent X-ray diffraction (XRD), we investigate how our synthesized phases can be turned into MSiO3 structures. Finally, the Goldschmidt rules for perovskites structures show that this might be an attractive way for new and nontoxic phases in the future.

Perovskites with a Twist: Strong In1+ Off-Centering in the Mixed-Valent CsInX3 (X = Cl, Br)

The search for new halide perovskites has recently expanded to the double perovskites A2M+M3+X6, which form in the 3D elpasolite structure with alternating M+ and M3+ octahedra. Here, we report the ternary mixed-valent indium compounds CsInX3 (X = Br, Cl). They adopt a tetragonal (space group I4/m) structure, which is a derivative of the charge-ordered double perovskite but with a twisted chain of octahedra along the c-axis. This twist is a result of the off-centering at the In+ site to accommodate its stereochemically active 5s2 lone pair, which induces a 45° rotation of all In3+ octahedra in the chain. This is a non-cooperative rotation that creates pentagonal pyramids in the structure and induces significant disorder, with extensive twinning, which results in partial occupation of different rotations of the same In3+ site. Temperature-dependent synchrotron X-ray diffraction reveals that both compounds form the cubic double perovskite structure at high temperature (Fm3̅m), thus demonstrating that the twist occurs on cooling from the melt. UV–vis spectroscopy reveals band gaps near 2.3–2.4 eV for the bromide and 3.0 eV for CsInCl3. High-pressure electrical transport measurements show significant enhancement in conductivity as pressure increases, indicating a narrowing of the band gap at high pressures. Temperature-dependent transport measurements at high pressure show uniformly semiconducting behavior, with no superconducting transition observed down to 2.4 K. The CsInX3 are unique inorganic halide double perovskites because they are based on a single metal, with CsInBr3 just the third bromide after the mixed metal compounds Cs2AgBiBr6 and Cs2AgTlBr6.

Thermal hysteresis control of VO2 (M) nanoparticles by Ti-F codoping

We have synthesized F-doped TixV1−xO2 nanoparticles by hydrothermal method and examined the structural phase transition behavior depending on the Ti and F contents. Differential scanning calorimetry and temperature-dependent X-ray diffraction analyses revealed that, by the codoping of Ti and F in VO2, both the critical phase transition temperature (Tc) and the thermal hysteresis width (ΔTc) are appropriately reduced. We have also examined the thermochromic behavior for the composite films prepared by casting F-doped TixV1−xO2 nanoparticles. The 2.0% F-doped Ti0.08V0.92O2 composite film exhibited a decreased Tc of approximately 20 °C and narrow ΔTc of below 2 °C. This result clearly shows the potential of F-doped TixV1−xO2 nanoparticles for sensitive optical device application.

Effect of Ti:ZnO Layer on the Phase Transition and the Optical Properties of VO2 Film

VO2 single-layer and VO2/Ti:ZnO bilayer thin film samples were stabilized on quartz substrates by using sputtering technique, and their structural, morphological and optical properties were investigated. The X-ray diffraction (XRD) results confirmed that VO2 stabilized in the mono-clinic structure in both the samples. Temperature-dependent XRD results demonstrated that the monoclinic-to-rutile structural phase transition temperature had shifted in bilayer sample as compared to VO2 single-layer sample. A uniform surface morphology was realized in both samples, as shown by atomic force microscopy. UV-vis spectroscopy results revealed a significant improvement in the optical transmission in the VO2/Ti:ZnO bilayer thin-film sample. Improved visible transmittance of bilayer sample identifies it as a good candidate for smart window applications.

Basic physical properties of cubic boron arsenide

Cubic boron arsenide (BAs) is an emerging semiconductor material with a record-high thermal conductivity subject to intensive research interest for its applications in electronics thermal management. However, many fundamental properties of BAs remain unexplored experimentally since high-quality BAs single crystals have only been obtained very recently. Here, we report the systematic experimental measurements of important physical properties of BAs, including the bandgap, optical refractive index, elastic modulus, shear modulus, Poisson's ratio, thermal expansion coefficient, and heat capacity. In particular, light absorption and Fabry–Pérot interference were used to measure an optical bandgap of 1.82 eV and a refractive index of 3.29 (657 nm) at room temperature. A picoultrasonic method, based on ultrafast optical pump probe spectroscopy, was used to measure a high elastic modulus of 326 GPa, which is twice that of silicon. Furthermore, temperature-dependent X-ray diffraction was used to measure a linear thermal expansion coefficient of 3.85 × 10−6 K−1; this value is very close to prototype semiconductors such as GaN, which underscores the promise of BAs for cooling high power and high frequency electronics. We also performed ab initio theory calculations and observed good agreement between the experimental and theoretical results. Importantly, this work aims to build a database (Table I) for the basic physical properties of BAs with the expectation that this semiconductor will inspire broad research and applications in electronics, photonics, and mechanics.

Structural changes in chlorine-substituted SbSI

The antimony sulphoiodide (SbSI) is considered a prospective and important ferroelectric material due to its unique properties below Curie temperature Tc. However, the fact that current practical applications require higher working temperatures has prompted new structural improvements that extend the ferroelectric state. In this ternary system, Tc is highly sensitive to any chemical modifications or stress. Therefore, one way to adjust the Tc is through selective substitution of the constituent elements. In this work, SbSI has been fractionally chlorine-substituted at the iodine site and examined using temperature-dependent x-ray diffraction and specific heat capacity methods. Although a considerable increase in Tc has been achieved, a more detailed analysis shows that the Tc increases with x from 0 to 0.2 and starts to decrease when x > 0.2. The maximum Tc increase in the range of x = 0–0.3 is ∼15.3%. The reverse behavior, from increase to decrease, is thoroughly discussed with reference to the previously published data on SbSI1-xClx compounds.

Structural, Photophysical, and Electronic Properties of CH3NH3PbCl3 Single Crystals

Methylammonium lead chloride (CH3NH3PbCl3 or MAPbCl3) single crystals were fabricated using the inverse temperature crystallization method, and their structural, photophysical, and electronic characteristics were studied using temperature dependent optical spectroscopy, X-ray diffraction (XRD), current-voltage, and Hall measurements. The changes in absorption and photoluminescence properties accompanied with structural changes in crystal lattice were studied within a broad temperature range of 300–20 K. XRD investigations reveal that phase changes took placed around 180 K and 175 K. At a temperature below 170 K, two different crystallographic phases were found to co-exist in the photoluminescence spectra. An asymmetric line shape with broad and weak shoulders near the absorption edges was observed in all of the major PL peaks. The weak shoulders are attributed to the missing chloride atoms on the crystal surface. The photoluminescence intensity of the crystals was strongly influenced by the environment, thereby indicating that the carrier recombination is affected by the physical desorption/absorption of gas molecules at the crystal surface. Moreover, vibronic replicas in the photoluminescence spectra at low temperature were observed for the first time. The origins of these replicas are attributed to the coupling between the vibrational/librational motions of the organic cations and the photoexcited electrons. Finally, the Hall and current-voltage measurements confirm that the crystal is an n-type semiconductor with a carrier concentration of ~2.63 × 1011 cm−3, a mobility of 4.14 cm2/V•s, and a conductivity of 1.8 × 10−8Ω−1 cm−1 under dark and room temperature conditions.

Large Batch Coating Process for Improving Thermal Stability of NCM622 Cathode

Nickel-rich cathodes such as LiNi0.6Co0.2Mn0.2O2 (NCM622) have received considerable attention as a result of their high delivered capacity. However, owing to high nickel content, NCM622 cathodes suffer from reduced cycle life and thermal stability compared to lower nickel content NCM523 and NCM333 cathodes. (1) Traditional approaches to coating and doping, including exploration of various coating (Al2O3, AlF3, SiO2) and dopant (Al3+, Mg2+, F-) species which either act as a passivating layer between the cathode surface and reactive electrolyte or to stabilize the structure of the material, have been employed in the past to improve the performance of Ni-rich cathodes. (2-9) This presentation demonstrates the effectiveness of a large batch (i.e. 500 grams) coating process to deposit either AlF3 or ZnO on NCM622 cathode material. The process is a dry mixing technique wherein the coating is applied using powerful mechanical energy to induce mechano-chemical reactions between the surface of NCM622 and AlF3 or ZnO. SEM imaging and EDX mapping illustrate that the size or morphology of the secondary particles is not affected by the process and the coating species is well-distributed among the bulk. Differential scanning calorimetry (DSC) and temperature-depending X-ray diffraction (XRD) were conducted to monitor the thermal stability of AlF3 and ZnO coated NCM622. Temperature-dependent XRD shows the structural degradation of the layered NCM622 structure to a non-layered rock-salt structure as a function of temperature. The peak positions in the DSC correspond directly to the c lattice parameter decreasing, which is caused by oxygen release, and the conversion to a rock-salt phase. These results indicate how AlF3 and ZnO coatings alter the collapse and phase transition of the delithiated NCM622 during heating. References (1) Liu, W.; Oh, P.; Liu, X.; Lee, M.-J.; Cho, W.; Chae, S.; Kim, Y.; Cho, J. Chem. Int. Ed. 2015, 54, 4440-4457. (2) Mohanty, D.; Dahlberg, K.; King, D. M.; David, L. A.; Sefat, A. S.; Wood, D. L.; Daniel, C.; Dhar, S.; Mahajan, V.; Lee, M.; Albano, F. Reports 2016, 6, 26532. (3) Han, B.; Paulauskas, T.; Key, B.; Peebles, C.; Park, J. S.; Klie, R. F.; Vaughey, J. T.; Dogan, F. ACS Appl. Mater. Interfaces 2017, 9, 14769-14778. (4) Liao, J.-Y.; Manthiram, A. Power Sources 2015, 282, 429-436. (5) Du, K.; Xie, H.; Hu, G.; Peng, Z.; Cao, Y.; Yu, F. ACS Appl. Mater. Interfaces 2016, 8, 17713-17720. (6) Zhou, P.; Zhang, Z.; Meng, H.; Lu, Y.; Cao, J.; Cheng, F.; Tao, Z.; Chen, J. Nanoscale 2016, 8, 19263-19269. (7) Woo, S.-U.; Yoon, C. S.; Amine, K.; Belharouak, I.; Sun, Y.-K. Electrochem. Soc. 2007, 154, A1005-A1009. (8) Hu, G.; Zhang, M.; Liang, L.; Peng, Z.; Du, K.; Cao, Y. Electrochimica Acta 2016, 190, 264-275. (9) Krishna Kumar, S.; Ghosh, S.; Ghosal, P.; Martha, S. K. Power Sources 2017, 356, 115-123.

LATP and LiCoPO4 thin film preparation – Illustrating interfacial issues on the way to all-phosphate SSBs

Thin film solid-state batteries are suitable model systems for the study of degradation phenomena in bulk solid-state batteries. As a necessary prerequisite for the construction of thin film batteries, high quality thin films of electrode materials and solid electrolytes have to be deposited. In this study, phosphate-based cathode (LiCoPO4) and solid electrolyte (Li1.5Al0.5Ti1.5(PO4)3) thin films were successfully deposited by pulsed laser deposition on silicon substrates. By temperature dependent grazing incidence X-ray diffraction measurements, the high quality of the deposited thin films was shown as well as crystallization temperatures were determined. Moreover, atomic force microscopy and scanning electron microscopy measurements were carried out for surface analysis, highlighting the surface smoothness of the films and unraveling the microstructure of the deposited LATP films, which contain grain boundaries. Further, a model interface composed of a thin layer of LiCoPO4 of varying thickness on top of Li1.5Al0.5Ti1.5(PO4)3 was used to corroborate information about changes in the chemical state of the materials as well as to track the inter-diffusion across the interface of all corresponding ionic species. This was done by X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry depth profiling. While well-defined interfaces were observed for unheated interfaces, significant inter-diffusion of transition metal ions was observed between heated LATP and LCP films. Despite inter-diffusion, no changes in the chemical states were observed at the interface, excluding significant phase transformations to compounds with altered oxidation states. Although stable phosphate-based materials were chosen for, both, electrolyte and active material to diminish known instabilities of the interface in this study, the need for interfacial layers arises to suppress the inter-diffusion, which may affect lithium ion transport across the interface. This work provides detailed insight into the complex problems of interfacial stability in all-solid-state batteries.

Reversible Thermochromism of Nickel(II) Complexes and Single-Crystal-to-Single-Crystal Transformation.

A molecular Ni(II)–NNN pincer complex (1) exhibited unprecedented reversible single-crystal-to-single-crystal transformation and color change upon heating and cooling due to a subtle change in the N–Ni(II) bond length and ligand conformation. UV–vis, thermogravimetric, differential scanning calorimetry, single-crystal structural data, temperature-dependent powder X-ray diffraction, and Raman and computational studies supported the structural change of the Ni(II) complex with temperature.

Asymmetric hysteresis in vanadium dioxide thin films

Vanadium dioxide is a material that undergoes phase changes and switches between metallic and insulating states, thereby producing dramatic changes in optical properties. This transition is a reversible but hysteretic process, which is investigated here as a function of atomic layer deposited film thickness. Increasing the thickness of vanadium dioxide films from 8.6 to 57 nm lead to an increase in hysteresis width. Thicker films develop two different slopes (steep and gradual) when cooling through the metal-insulator transition, where the steep transition matches that of the heating cycle of the transition. This asymmetry in the hysteresis is apparent and similar in both the electrical and optical measurements. Temperature-dependent Raman spectroscopy and temperature-dependent x-ray diffraction confirm the same anomaly suggesting a structural dependence on hysteretic shape. Transmission electron microscopy identifies texturing and faceting in-plane, especially along the surface of these films, and confirms the x-ray diffraction data. Identifying this facet texturing is valuable for film growth as well as for applications, such as logic and memory systems, that utilize the hysteretic nature of vanadium dioxide.

An Old Dog with New Tricks - Additions to the Cesium Lithium Chloride System: Cs3Li2Cl5 and the Hydrated Cs3LiCl4 · 4H2O.

The CsCl/LiCl system has been studied for over a century now. Numerous phases have been predicted - only three have hitherto been found. We present the synthesis and single-crystal structure of the cesium lithium pentachloride Cs3Li2Cl5, predicted earlier but with a different structure. The anhydrous new phase readily reacts to Cs3LiCl4 · 4H2O in air. The tetrahydrate can also be obtained through the simplest, most inexpensive and green synthesis possible: an immediate, room-temperature mechanosynthesis from only CsCl and LiCl (3 : 1) in air. Differential scanning calorimetry (DSC) and thermogravimetric analyses (TGA), as well as in situ temperature-dependent powder X-ray diffraction studies on this second ever reported ternary alkali chloride hydrate allowed for a revision of the CsCl/LiCl phase diagram. Density of states and total energy calculations further elucidate the new alkali chlorides and update the relative stability of previous structure predictions.

Tailoring the structural and magnetic properties of epitaxially growth Ni–Mn–Ga magnetic films by Fe addition

ABSTRACTNi–Mn–Ga is one of the major magnetic materials in Heusler family due to its interesting magnetic and structural properties. In this study, the epitaxial 200 nm thin films are prepared by magnetron co-sputtering from a NiMnGa alloy target and an elemental Fe target on MgO substrates. The epitaxial growth mechanism is pursued and controlled by ex-situ X-ray refractometry (XRR) and X-ray diffraction (XRD) measurements. On account of analysing the physical parameters, temperature-dependent X-ray diffraction (XRD), transmission electron (TEM), scanning electron microscopy (SEM), temperature dependence of resistivity (ρ(T)), atomic force microscopy (AFM) and magnetisation (MOKE) measurements are performed systematically for different Fe contents. Structural and magnetic measurements show that the NiMnGa parent thin film transitions could be adjusted from martensitic to austenitic phases by adding Fe element from high temperature to the vicinity of room temperature. Measurements are carried out to get a further structural characterisation of 14M martensite which was found to consist of nano-twinned structures. The epitaxial growth and the stable coexisting austenite and martensitic phases for the thin film of the NiMnGa chemical composition are shown near room temperature. Due to this feature, magnetic thin films could increase the likelihood of possible room temperature using thin film applications.

Strong Electron-Phonon Coupling in the Excitonic Insulator Ta2NiSe5.

An excitonic insulating (EI) state is a fantastic correlated electron phase in condensed matter physics, driven by screened electron-hole interaction. Ta2NiSe5 is an excitonic insulator with a critical temperature (TC) of 328 K. In the current study, temperature-dependent Raman spectroscopy is used to investigate the phonon vibrations in Ta2NiSe5. The following observations were made: (1) an abnormal blue shift around TC is observed, which originates from the monoclinic to orthorhombic structural phase transition; (2) the splitting of a mode and two new Raman modes at 147 and 235 cm-1 have been observed with the formation of an EI state. With the help of first-principles calculations and temperature-dependent X-ray diffraction (XRD) experiments, it is found that the TaSe6 octahedra are "frozen" and the NiSe4 tetrahedra are greatly distorted below TC. Thus, it seems that the distortion of NiSe4 tetrahedra plays an important role in the strong electron-phonon coupling (EPC) in Ta2NiSe5, while the strong EPC, coupled with electron-hole interaction, opens the energy gap to form the EI state in Ta2NiSe5.

Local Structure and Influence of Sb Substitution on the Structure-Transport Properties in AgBiSe2.

Owing to their intrinsically low thermal conductivity and chemical diversity, materials within the I-V-VI2 family, and especially AgBiSe2, have recently attracted interest as promising thermoelectric materials. However, further investigations are needed in order to develop a more fundamental understanding of the origin of the low thermal conductivity in AgBiSe2, to evaluate possible stereochemical activity of the 6s2 lone pair of Bi3+, and to further elaborate on chemical design approaches for influencing the occurring phase transitions. In this work, a combination of temperature-dependent X-ray diffraction, Rietveld refinements of laboratory X-ray diffraction data, and pair distribution function analyses of synchrotron X-ray diffraction data is used to tackle the influence of Sb substitution within AgBi1-xSbxSe2 (0 ⩽ x ⩽ 0.15) on the phase transitions, local distortions, and off-centering of the structure. This work shows that, similar to other lone-pair-containing materials, local off-centering and distortions can be found in AgBiSe2. Furthermore, electronic and thermal transport measurements, in combination with the modeling of point-defect scattering, highlight the importance of structural characterizations toward understanding changes induced by elemental substitutions. This work provides new insights into the structure-transport correlations of the thermoelectric AgBiSe2.

Characterization of structural transitions and lattice dynamics of hybrid organic–inorganic perovskite CH3NH3PbI3**Project supported by the National Natural Science Foundation of China (Grant No. 11774419) and the Ministry of Science and Technology of China (Grant Nos. 2016YFA0300504 and 2017YFA0302904).

By combining temperature-dependent x-ray diffraction (XRD) with temperature-dependent Raman scattering, we have characterized the structural transitions and lattice dynamics of the hybrid organic–inorganic perovskite CH3NH3PbI3. The XRD measurements cover distinct phases between 15 K and 370 K and demonstrate a general positive thermal expansion. Clear anomalies are found around the transition temperatures. The temperature evolution of the lattice constants reveals that the transition at 160 K/330 K is of the first-/second-order type. Raman measurements uncover three strong low-frequency modes, which can be ascribed to the vibration of the Pb/I atoms. The temperature evolution of the modes clearly catches these transitions at 160 K and 330 K, and confirms the transition types, which are exactly consistent with the XRD results. The present study may set an experimental basis to understand the high conversion efficiency in methylammonium lead iodide.

Tunable spin-state bistability in a spin crossover molecular complex

The spin crossover (SCO) transitions at both the surface and over the entire volume of the [Fe{H2B(pz)2}2(bipy)] polycrystalline films on Al2O3 substrates have been studied, where pz = pyrazol-1-yl and bipy = 2,2′-bipyridine. For [Fe{H2B(pz)2}2(bipy)] films of hundreds of nm thick, magnetometry and x-ray absorption spectroscopy measurements show thermal hysteresis in the SCO transition with temperature, although the transition in bulk [Fe{H2B(pz)2}2(bipy)] occurs in a non-hysteretic fashion at 157 K. While the size of the crystallites in those films are similar, the hysteresis becomes more prominent in thinner films, indicating a significant effect of the [Fe{H2B(pz)2}2(bipy)]/Al2O3 interface. Bistability of spin states, which can be inferred from the thermal hysteresis, was directly observed using temperature-dependent x-ray diffraction; the crystallites behave as spin-state domains that coexist during the transition. The difference between the spin state of molecules at the surface of the [Fe{H2B(pz)2}2(bipy)] films and that of the molecules within the films, during the thermal cycle, indicates that both cooperative (intermolecular) effects and coordination are implicated in perturbations to the SCO transition.

Temperature-Induced Single-Crystal-to-Single-Crystal Transformations with Consequential Changes in the Magnetic Properties of Fe(III) Complexes.

The present article deals with an one-to-one structure–property correspondence of a dinuclear iron complex, [Dipic(H2O)FeOH]2·H2O (1) (Dipic = pyridine-2,6-dicarboxylic acid). Variable-temperature X-ray single-crystal structural analysis confirms a phase transition of complex 1 to complex 2 ([Dipic(H2O)FeOH]2) at 120 °C. Further, single-crystal-to-single-crystal (SCSC) transformation was monitored by temperature-dependent single crystal X-ray diffraction, powder X-ray diffraction, time-dependent Fourier-transform infrared spectroscopy, and differential scanning calorimetry. SCSC transformation brings the change in space group of single crystal. Complex 1 crystallizes in the C2/c space group, whereas complex 2 crystallizes in the Pi̅ space group. SCSC transformation brings the change in packing diagram as well. Complex 1 shows two-dimensional network through H-bonding, whereas the packing diagram of complex 2 shows a zigzag-like arrangement. Phase transformation not only fetches structural changes but also in the magnetic properties. Difference in Fe–O–Fe bond angles of two complexes creates notable variation in their antiferromagnetic interactions with adjacent metal centers.

Mechanosynthesis of the Whole Y1-xBixMn1-xFexO3 Perovskite System: Structural Characterization and Study of Phase Transitions.

Perovskite BiFeO3 and YMnO3 are both multiferroic materials with distinctive magnetoelectric coupling phenomena. Owing to this, the Y1−xBix Mn1−xFexO3 solid solution seems to be a promising system, though poorly studied. This is due to the metastable nature of the orthorhombic perovskite phase of YMnO3 at ambient pressure, and to the complexity of obtaining pure rhombohedral phases for BiFeO3-rich compositions. In this work, nanocrystalline powders across the whole perovskite system were prepared for the first time by mechanosynthesis in a high-energy planetary mill, avoiding high pressure and temperature routes. Thermal decomposition temperatures were determined, and structural characterization was carried out by X-ray powder diffraction and Raman spectroscopy on thermally treated samples of enhanced crystallinity. Two polymorphic phases with orthorhombic Pnma and rhombohedral R3c h symmetries, and their coexistence over a wide compositional range were found. A gradual evolution of the lattice parameters with the composition was revealed for both phases, which suggests the existence of two continuous solid solutions. Following bibliographic data for BiFeO3, first order ferroic phase transitions were located by differential thermal analysis in compositions with x ≥ 0.9. Furthermore, an orthorhombic-rhombohedral structural evolution across the ferroelectric transition was characterized with temperature-dependent X-ray diffraction.

Temperature Dependent X-Ray Diffraction in K2FeP2O7 Pyrophosphate: Polymorphism and Preliminary Electrochemistry Towards Li, Na, and K Ion Intercalation

Phosphate-based polyanionic materials have changed the era of commercial secondary batteries after the discovery of triphylite LiFePO4 in 1997. Such materials are encouraged due to its open and robust framework, chemical/ thermal stability, tunable redox potential and desirable electrochemical performance. Several phosphate-based electrode materials have been reported for lithium (Li+) and sodium (Na+) insertion to develop lithium-ion batteries (LIB) and sodium-ion batteries (SIB). The fact that Na+ can't be intercalated into graphite (which is widely used negative electrode for battery application) has diverted researcher’s attention toward potassium-ion batteries (KIB).[1] Also regarding redox center, polyanion chemistry has only created a basis for iron-based compounds to be used as cathode in LIB. The elemental abundance and non-toxic nature has encouraged battery community to explore Fe-based battery insertion materials. The progressive studies on pyrophosphate polyanionic cathodes has developed them as potential compounds for next generation batteries. Promising electrochemical properties of Li2FeP2O7 and Na2FeP2O7 has motivated us to study potassium-based analogues K2FeP2O7. [2] K2FeP2O7 was synthesized using solid state and combustion method. In particular, at first this phospho-anionic compound was prepared by solution combustion synthesis using low cost Fe(III) based precursors [Fe(NO3)3.9H2O]. Fe(NO3)3 was dissolved in distilled water followed by the addition of ascorbic acid to convert all Fe (III) into Fe(II). Then stoichiometric amount of KH2PO4 and combustion fuel like citric acid or urea were added. These solutions were heated at 110 ºC to trigger an exothermic reaction developing an intermediate complex. Upon annealing this complex at high temperature (500, 600 and 700 ºC), similar unknown phase members at all three temperature were obtained inferred from X-ray diffraction technique (XRD). To validate the unknown phase, stoichiometric amounts of FeC2O4.2H2O and KH2PO4 were added and ball milled. Similarly, the ball milled intermediate were annealed at high temperature (500, 600 and 700 ºC). The synthesized phases were analyzed using XRD, 500 and 600 ºC compounds are same as obtained by solution combustion synthesis. Interestingly, 700 ºC synthesized phase was found to crystalize into the structure type of Ca2MgSi2O7 (previously reported by Keates et al. K2FeP2O7).[3] Such simple observation of change in phase observed from XRD profile gave us a clue of polymorphism phenomenon which was further cross-validated by temperature dependent XRD in K2FeP2O7 obtained at 500 ºC. High temperature XRD results clearly shows the existence of two stable phases (Figure 1. in the middle), low temperature (500 ºC) one can be assigned as β-K2FeP2O7 and high temperature (700 º C) can be assigned as α-K2FeP2O7. Structure determination of β-K2FeP2O7 is in its almost final stage with monoclinic (P21/c) crystal structure. α-K2FeP2O7 crystalizes into tetragonal crystal system of P-421 m space group. Pyrophosphate compounds offer a robust three-dimensional P2O7 4--framework with multiple sites for alkali ions and existence of polymorphism in such compounds is clear in the present case. Polymorphism arises due to the different angular orientation between the two phosphate units. Here, we report the electrochemistry of the β-K2FeP2O7 and α-K2FeP2O7 with respect to Li+, Na+ and K+. Galvanostatic charge-discharge profile of β-K2FeP2O7 and α-K2FeP2O7 is compared and shown in the left and right side of the figure respectively. Both materials have low insertion voltages which is lesser than 3 V. This low working voltage is possibly coming due the existence of FeO4 tetrahedral environment in the crystal structure which leads to the weak inductive effects as compared to FeO6 octahedral environment during Fe2+/Fe3+ redox mechanism.[4] Seeing these unpromising intercalation chemistry both polymorphs were subjected to study the ac impedance conductivity by varying the temperature from room temperature to 500 ºC in neutral atmosphere. β-K2FeP2O7 polymorph was found to have conductivity ~10-6 S cm-1 at 300 ºC and 10-3 S cm-1 at 400 ºC. However, α-K2FeP2O7 was found to have very poor conductivity experimentally which is in sync with BVSE energy barrier of 1.08 eV. Thus the current work will summarize recent findings in ‘pyrophosphate’ class of battery materials capable of reversible insertion of Li+ into β-K2FeP2O7 along with high temperature conductivity measurements using AC-impedance analyzer.