High-temperature Orthorhombic Phase Research Articles

Luminescence properties and phase transformation of broadband NIR emitting A2(WO4)3:Cr3+ (A=Al3+, Sc3+) phosphors toward NIR spectroscopy applications

The synthesis, structural, and luminescence properties have been carried out for Cr3+-activated Al2(WO4)3 (AWO) and Sc2(WO4)3 (SWO) phosphors for application in pc-NIR LED. Upon blue excitation, these compounds are capable of exhibiting broadband NIR emission stems primarily from 4T2→4A2 transition in the range of 670–1200 nm (maxima ∼808 nm, FWHM ∼140 nm) for AWO:Cr and of 700–1300 nm (maxima ∼870 nm, FWHM ∼164 nm) for SWO:Cr. The significant shift of NIR emission is attributed to the substitution of AlO6 with larger ScO6 octahedrons. To gain insight into the luminescence the crystal field strength, Racah parameters, nephelauxetic effect, and electron-phonon coupling have been analyzed based on spectroscopic results. The electron-phonon coupling parameter S for SWO:Cr was determined to be 11.5, twice as large as that for AWO:Cr, which is in accordance with its strong thermal quenching. The abrupt changes occurring at 275 K in temperature-dependent luminescence spectra and decay lifetime of AWO:Cr is associated with temperature-driven phase transformation from low-temperature monoclinic to high-temperature orthorhombic phase. Pressure induced amorphization of AWO:Cr at pressures higher than 25 kbar was confirmed by employing high pressure evolution of Raman spectra. A high-power NIR pc-LED, fabricated by coating AWO:0.04Cr on a commercial 470 nm LED chip, shows good performance with an output power of 17.1 mW driven by a current of 320 mA, revealing potential application of studied materials for NIR light source.

Methylhydrazinium]2PbCl4, a Two-Dimensional Perovskite with Polar and Modulated Phases

Two-dimensional (2D) lead halide perovskites are a family of materials at the heart of solar cell, light-emitting diode, and photodetector technologies. This perspective leads to a number of synthetic efforts toward materials of this class, including those with prescribed polar architectures. The methylhydrazinium (MHy+) cation was recently presumed to have an unusual capacity to generate non-centrosymmetric perovskite phases, despite its intrinsically nonchiral structure. Here, we witness this effect once again in the case of the Ruddlesden–Popper perovskite phase of formula MHy2PbCl4. MHy2PbCl4 features three temperature-dependent crystal phases, with two first-order phase transitions at T1 = 338.2 K (331.8 K) and T2 = 224.0 K (205.2 K) observed in the heating (cooling) modes, respectively. Observed transitions involve a transformation from high-temperature orthorhombic phase I, with the centrosymmetric space group Pmmn, through the room-temperature modulated phase II, with the average structure being isostructural to I, to the low-temperature monoclinic phase III, with non-centrosymmetric space group P21. The intermediate phase II is a rare example of a modulated structure in 2D perovskites, with Pmmn(00γ)s00 superspace symmetry and modulation vector q ≅ 0.25c*. MHy2PbCl4 beats the previous record of MHy2PbBr4 in terms of the shortest inorganic interlayer distance in 2D perovskites (8.79 Å at 350 K vs 8.66 Å at 295 K, respectively). The characteristics of phase transitions are explored with differential scanning calorimetry, dielectric, and Raman spectroscopies. The non-centrosymmetry of phase III is confirmed with second harmonic generation (SHG) measurements, and polarity is demonstrated by the pyroelectric effect. MHy2PbCl4 also exhibits thermochromism, with the photoluminescence (PL) color changing from purplish-blue at 80 K to bluish-green at 230 K. The demonstration of polar characteristics for one more member of the methylhydrazinium perovskites settles a debate about whether this approach can present value for the crystal engineering of acentric solids similar to that which was recently adopted by a so-called fluorine substitution effect.

High-temperature orthorhombic phase of Cu2HgGeS4: Electronic structure and principal optical constants as evidenced from the experiment and theory

We report on successful synthesis of single-phase high-temperature (HT) orthorhombic modification of Cu2HgGeS4 and studies of its electronic and optical properties from a viewpoint of experiment and theory. In particular, we have made measurements of the XPS core-level and valence-band spectra of the HT-Cu2HgGeS4 modification and calculated density of states. The calculations are carried out within a framework of density functional theory and using varieties of techniques. We have realized that the best agreement of the theory and experiment is observed when we use in the calculations the modified Becke-Johnson (mBJ) functional as refined by Tran-Blaha and consider also the Hubbard correction parameter U and spin-orbit coupling (SOC). The calculations demonstrate that main contributions of sulfur 3p states occur near the topmost and upper part of the HT-Cu2HgGeS4 valence band, copper 3d states dominate in its central part, the lower part is prevailed by contributions of mercury 6s and germanium 4p states, whereas near the valence band bottom the biggest contributions come from mercury 5d states. The theoretical predictions are verified experimentally. The calculations reveal that the HT-Cu2HgGeS4 modification is a direct band gap semiconductor. The principal optical constants are elucidated following first-principles calculations of the quaternary sulfide under consideration.

Temperature dependence of three-dimensional domain wall arrangement in ferroelectric K0.9Na0.1NbO3 epitaxial thin films

The three-dimensional arrangement and orientation of domain walls in ferroelectric K0.9Na0.1NbO3/(110)NdScO3 epitaxial thin films were investigated at different temperatures both experimentally by means of piezoresponse force microscopy and three-dimensional x-ray diffraction and theoretically by three-dimensional phase-field simulations. At room temperature, a well-ordered herringbone-like domain pattern appears in which there is a periodic arrangement of a1a2/MC monoclinic phases. Four different types of domain walls are observed, which can be characterized by out-of-plane tilt angles of ±45° and in-plane twist angles of ±21°. For the orthorhombic high-temperature phase, a periodic a1/a2 stripe domain pattern with exclusive in-plane polarization is formed. Here, two different types of domain walls are observed, both of them having a fixed out-of-plane domain wall angle of 90° but distinguished by different in-plane twist angles of ±45°. The experimental results are fully consistent with three-dimensional phase-field simulations using anisotropic misfit strains. The qualitative agreement between the experiment and the theory applies, in particular, to the wide phase transition range between about 180 °C and 260 °C. In this temperature range, a complex interplay of coexisting monoclinic a1a2/MC and orthorhombic a1/a2 phases takes place.

Crystal structures and rotational dynamics of a two-dimensional metal halide perovskite (OA)2PbI4.

The extended charge carrier lifetime in metal halide perovskites is responsible for their excellent optoelectronic properties. Recent studies indicate that the superb device performance in these materials is intimately related to the organic cation dynamics. Here, we focus on the investigation of the two-dimensional hybrid perovskite, (C8H17NH3)2PbI4 (henceforth, OA+ = C8H17NH3 +). Using elastic and quasielastic neutron scattering techniques and group theoretical analysis, we studied the structural phase transitions and rotational modes of the C8H17NH3 + cation in (OA)2PbI4. Our results show that, in the high-temperature orthorhombic (T > 310 K) phase, the OA+ cation exhibits a combination of a twofold rotation of the NH3-CH2 head group about the crystal c-axis with a characteristic relaxation time of ∼6.2 ps, threefold rotations (C3) of NH3 and CH3 terminal groups, and slow librations of the other atoms. Contrastingly, only the C3 rotation is present in the intermediate-temperature orthorhombic (238 K < T < 310 K) and low-temperature monoclinic (T < 238 K) phases.

Electronic, structural, and optical properties of Y2WO6, a host material for inorganic phosphors

Optimization by first principles DFT-based electronic structure methods of the crystal structures for the five polymorphs of Y2WO6 reported in the literature yields results in good agreement with those determined experimentally by X-ray diffraction. The monoclinic P2/c phase appears to be the most stable one at ambient conditions, although high temperature orthorhombic phases with larger molar volumes could be favoured upon replacement of Y3+ cations by larger Ln3+ ones, and hence, provide plausible structures for Y2WO6:Ln3+ phosphors at ambient conditions. For all polymorphs the top of the valence band is dominated by O2p orbitals with a relatively narrow WO6-centred conduction band appearing just below a broad Y4d-centred band. Insertion energies for Eu3+ replacing Y3+ are estimated to be in the range of 3–4 eV per cation, with the smaller values corresponding to substitutions into the larger octacoordinated Y3+ sites.

Structural evolution of La2Ti2O7 at elevated temperatures.

Structural evolution of a La2Ti2O7 ferroelectric compound possessing perovskite-type slabs at elevated temperatures was investigated using the single-crystal X-ray diffraction technique. The monoclinic low-temperature phase (L) transformed into the orthorhombic high-temperature phase (H) via an incommensurately modulated phase (IC) between ∼989 and ∼1080 K. The L-IC transition was considered to be of the first order, with the L+IC two-phase co-existing region between ∼989 and ∼1027 K. The structure of IC was determined from the (3+1)-dimensional superspace representation with a modulation vector q = αao (α ≃ 0.49), where ao is the a-axis vector of the basic cell. The structural modulation originated from the variation of the tilt angle of the TiO6 octahedra in the perovskite-type slab in association with small positional displacements of La atoms. The IC-H transition took place at ∼1080 K and was close to the second order. During the IC-H transition, nanoscale flat plate domains having either a cell twin of the L-type structural modules or a cell twin of the alternating H- and L-type structural modules began to appear in the approximant structure of IC. The thickness of the flat plate domains then grew rapidly along the modulation vector in proportion to (½ - α)-1 as α approached ½ with decreasing temperature. In the two-phase L+IC co-existing region, the IC phase consisting of the two types of cell twins was gradually replaced with the low-temperature monoclinic phase L, which is not cell twinned but rather twinned macroscopically by the L-type structural modules.

Synthesis and X-ray Diffraction Analysis of Cu2CdGexSn1 – xSe4 Solid Solutions

The high-temperature orthorhombic phase of Cu2CdGeSe4 quaternary compound, tetragonal phase of Cu2CdSnSe4 compound, and Cu2CdGexSn1 – xSe4 solid solutions have been grown from elementary components Cu, Cd, Ge, Sn, and Se by one-temperature synthesis. The unit-cell parameters of the grown compounds and solid solutions have been determined by X-ray diffraction analysis at room temperature. It is shown that a continuous series of solid solutions, based on Cu2CdSnSe4 compound, is formed in the Cu2CdGexSn1 – xSe4 system in the range of 0 ≤ х ≤ 0.8, and the Cu2CdGexSn1 – xSe4 system at x ≥ 0.9 is an orthorhombic crystalline phase.

The effect of hydrostatic pressure on magnetostructural transition in ferromagnetic cobaltite: Pr0.6Sr0.4CoO3

The ambient pressure temperature dependence of the magnetization (M) in P1-xSrxCoO3 series (x = 0.4, 0.5, 0.55 and 0.6) indicates the onset of ferromagnetic ordering at high temperatures (TC = 214-226K) and a step-like anomaly at lower temperatures (TS = 69-120 K). In this report we present the pressure dependence of magnetization and zero-field thermal expansion studies for a representative sample x = 0.4. The step-like anomaly in M(T) around TS = 69 K is accompanied by a pronounced peak in thermal expansion coefficient. The application of hydrostatic pressure of P = 1.16 GPa increases TS by 15 K, which is much larger than the increase of the ferromagnetic Curie temperature (ΔTC = 4 K) under the same pressure. The anomalous increase of TS under pressure is attributed to the occurrence of magnetization rotation at a temperature higher than TS = 69 K triggered by the stabilization of low temperature tetragonal phase at the expense of high temperature orthorhombic phase.

Understanding How Bonding Controls Strength Anisotropy in Hard Materials by Comparing the High-Pressure Behavior of Orthorhombic and Tetragonal Tungsten Monoboride

In this work, we investigate the high-pressure behavior of the stabilized high-temperature (HT) orthorhombic phase of WB using radial X-ray diffraction in a diamond-anvil cell at room temperature. The experiments were performed under nonhydrostatic compression up to 52 GPa. For comparison, the low-temperature (LT) tetragonal phase of WB was also compressed nonhydrostatically to 36 GPa to explore structurally induced changes to its mechanical properties. Although our microindentation hardness tests indicate that the HT WB possesses slightly higher hardness, synchrotron-based high-pressure compression data yield significant distinct incompressibilities. The ambient pressure bulk modulus of the HT phase of WB is 341 ± 5 GPa obtained by using the second-order Birch–Murnaghan equation of state, while for the LT phase of WB the incompressibility increased to 381 ± 3 GPa. The elastically supported differential stress was measured in a lattice-specific manner and analyzed by using lattice strain theory. Greater s...

The Crystal Structures of Pb5Sb4S11 (Boulangerite) – A Phase Transition Explains Seemingly Contradictory Structure Models

In the literature, many seemingly contradictory structure models of boulangerite, ideally Pb5Sb4S11, and the related mineral falkmanite have been reported. These can be explained by a phase transition between a low‐temperature (LT) modification (P21/c, a ≈ 8.07 Å, b ≈ 23.5 Å, c ≈ 21.6 Å, β ≈ 100.7°) and a disordered high‐temperature (HT) modification [Pnma, a = 23.530(2) Å; b = 4.0385(8) Å and c = 21.273(2) Å, R1(obs) = 0.039] that occurs above ca. 350–400 °C. The almost completely ordered distribution of lead and antimony cations in the LT modification involves a superstructure; the corresponding reflections are obvious in electron and X‐ray diffraction patterns. Partial ordering may render them more or less diffuse. They vanish upon heating above the transition temperature, also for originally ordered natural boulangerite (from Trepča, Kosovo), and appear again after annealing below the phase transition temperature (e.g. at 330 °C). The HT phase is characterized by pronounced cation disorder. Cation ordering can be described by group‐subgroup relationships which may also serve for a unified description of various structure models from literature. Starting from the orthorhombic HT phase, a translationengleiche followed by an isomorphous symmetry reduction yields the degrees of freedom required for cation ordering. The non‐standard setting A21/d11 is optimal for a unified comparison of all known structure models. Varying degrees of ordering reported in literature and observed for various new samples suggest that the transition is kinetically controlled. These relationships also contribute to the long‐lasting discussion about the nomenclature of minerals with the sum formula Pb5Sb4S11 or very similar ones. A precise structure determination of a single crystal (grown by chemical vapor transport) using synchrotron radiation along with detailed electron diffraction investigations and high‐resolution electron microscopy confirms the statistically disordered structure model of the quenched high‐temperature phase. Bond‐valence sums (BVS) and interatomic distances confirm the interpretation of cation split positions as the superposition of the corresponding independent cation positions in the LT phase. Measurements of transport properties indicate very low n‐type electrical and thermal conductivities for both modifications. The order‐disorder transition significantly affects the Seebeck coefficient.

Monoclinic-to-orthorhombic phase transition of the hexamethylenetetramine-2-methylbenzoic acid (1/2) cocrystal with temperature-dependent dynamic molecular disorder.

As a function of temperature, the hexamethylenetetramine-2-methylbenzoic acid (1/2) cocrystal, C6H12N4·2C8H8O2, undergoes a reversible structural phase transition. The orthorhombic high-temperature phase in the space group Pccn has been studied in the temperature range between 165 and 300 K. At 164 K, a t2 phase transition to the monoclinic subgroup P21/c space group occurs; the resulting twinned low-temperature phase was investigated in the temperature range between 164 and 100 K. The domains in the pseudomerohedral twin are related by a twofold rotation corresponding to the matrix (100/0-10/00-1. Systematic absence violations represent a sensitive criterium for the decision about the correct space-group assignment at each temperature. The fractional volume contributions of the minor twin domain in the low-temperature phase increases in the order 0.259 (2) → 0.318 (2) → 0.336 (2) → 0.341 (3) as the temperature increases in the order 150 → 160 → 163 → 164 K. The transformation occurs between the nonpolar point group mmm and the nonpolar point group 2/m, and corresponds to a ferroelastic transition or to a t2 structural phase transition. The asymmetric unit of the low-temperature phase consists of two hexamethylenetetramine molecules and four molecules of 2-methylbenzoic acid; it is smaller by a factor of 2 in the high-temperature phase and contains two half molecules of hexamethylenetetramine, which sit across twofold axes, and two molecules of the organic acid. In both phases, the hexamethylenetetramine residue and two benzoic acid molecules form a three-molecule aggregate; the low-temperature phase contains two of these aggregates in general positions, whereas they are situated on a crystallographic twofold axis in the high-temperature phase. In both phases, one of these three-molecule aggregates is disordered. For this disordered unit, the ratio between the major and minor conformer increases upon cooling from 0.567 (7):0.433 (7) at 170 K via 0.674 (6):0.326 (6) and 0.808 (5):0.192 (5) at 160 K to 0.803 (6):0.197 (6) and 0.900 (4):0.100 (4) at 150 K, indicating temperature-dependent dynamic molecular disorder. Even upon further cooling to 100 K, the disorder is retained in principle, albeit with very low site occupancies for the minor conformer.

Preparation and characterization of orthorhombic Fe2(MoO4)3 and first-principle study of its negative thermal expansion properties

Monoclinic Fe2(MoO4)3 sample is synthesized by the hydrothermal method, and characterized via high temperature X-ray diffraction and thermogravimetric-differential scanning calorimetry. It is observed that the reversible phase transition between the low-temperature monoclinic and high-temperature orthorhombic phases occurs at about 510 ℃. The cell parameters at different temperatures are calculated by the Rietveld refinement method. In a temperature range from 25 ℃ to 400 ℃, the a, b and c crystallographic axes with the monoclinic phase gradually expand. On the other hand, in a temperature range from 530 ℃ to 710 ℃, the orthorhombic phase exhibits a negative thermal expansion (NTE) behavior, in which the b and c axes gradually contract but the a axis first contracts and then expands a little. Atomic and electronic structures are investigated using first-principle calculation. Results indicate that the Mo-O bonds are much stronger than the Fe-O bonds in Fe2(MoO4)_{3} and the MoO4 tetrahedrons are more rigidly than FeO6 octahedrons. To reveal the relationship between NTE and polyhedral distortion, the phonon density of state of Fe2(MoO4)3 is calculated using the ab initio method. The experimental Raman spectrum positions can be identified in the calculated dispersion of the total phonon density of states (DOS). Meanwhile, by calculating the Grneisen parameters for phonon branches at point, the optical branch with the lowest vibration frequency is believed to have the largest negative Grneisen parameter. Furthermore, we analyze the vibrational behaviors of atoms, and find that oxygen atoms have different vibrational eigenvectors from Fe or Mo atoms. and more obvious amplitudes than Fe or Mo atoms. Therefore, it is concluded that the transverse vibration of the oxygen bridge atom between the MoO4 tetrahedron and FeO6 octahedron, the soft distortion of FeO6 octahedrons, and the rigid rotation of MoO4 tetrahedrons jointly lead to the negative thermal expansion of Fe2(MoO4)3,.

Charge transfer induced negative thermal expansion in perovskite BiNiO3

To clarify the mechanism of negative thermal expansion in perovskite BiNiO3, the structural, magnetic, and electronic properties of BiNiO3 in high-temperature orthorhombic and low-temperature triclinic phases are investigated within first-principles calculations. The optimized structures are ferromagnetic (FM) with 1.732μB/Ni adopting orthorhombic configuration and G-type antiferromagnetic (G-AFM) in triclinic phase. The electronic density of states (DOSs) and calculations of band valence sums (BVSs) show that the oxidation states are high-spin 3d7:t2g5eg2 of nickel in orthorhombic with Bi3+Ni3+O3, and an intermetallic charge transfer between Ni and Bi ions leading to Bi3+0.5Bi5+0.5Ni2+O3 of triclinic phase. Shrinkage about ∼1.07% in cell volume is attributed to the release of stress induced by changes of Ni–O and Bi–O bonds, which are corresponding to the changes of oxidation states in Bi and Ni ions.

Structural and magnetic phase separation of complex half-doped manganites by the example of Sm0.32Pr0.18Sr0.5MnO3

High-resolution neutron powder diffraction is used to investigate the phase separation and microscopic nature of the magnetoresistance in complex half-doped manganites. Investigations are performed by the example of Sm0.32Pr0.18Sr0.5MnO3 exhibiting the colossal magnetoresistance effect. The structural transition from a high-temperature orthorhombic Pbnm phase to a mixed phase containing two (orthorhombic Pbnm and monoclinic P21/m) phases is revealed. A gradual decrease in the amount of the orthorhombic phase and, consequently, an increase in the amount of the monoclinic phase are observed with decreasing temperature. Redistribution of the two phases ceases below T ≈ 100 K. Low-temperature analysis of the magnetic contribution to the experimental neutron-diffraction patterns indicates that the magnetic ground state of the compound under study is phase-separated and corresponds to a mixed state involving three magnetic phases formed at different temperatures: a ferromagnetic phase, A- type antiferromagnetic, and CE- type charge ordered antiferromagnetic phases with charge-orbital ordering. Ferromagnetic ordering arises near room temperature in the orthorhombic Pbnm phase and manifests itself only until helium temperatures. The A- and CE-types antiferromagnetic states (T N ≈ 170 and 120 K, respectively) appear in the monoclinic crystallographic phase.

Phase-Transition Mechanism Investigations in Monoclinic Li2FeSiO4 Orthosilicate Cathode

Lithium transition metal silicates Li2 M SiO4 ( M = Fe, Mn, Co, etc.)1 , 2, well-known for their 2-Li theoretical capacity, draw increasingly more attention. In this text, we propose a hydrothermal method3 to obtain phase-pure nanoscale lithium iron silicate Li2FeSiO4 for probing its electrochemical performance as Li-ion battery cathode. Two types of Li2FeSiO4 crystals are prepared, namely low-temperature monoclinic phase and high-temperature orthorhombic phase and their electrochemical performance is found to be strongly structure-depended. As a result, the electrochemistry-structure relationships are systematically investigated using in-situ synchrotron XRD/XANES characterizations and first-principles calculations. The results demonstrate that there are obvious phase transitions for the metastable monoclinic Li2FeSiO4 electrode upon cycling4 and the Li ion has poor diffusion kinetics with activation energy of minimum 0.80 eV in both structures. Furthermore the charge compensation mechanism is discussed by XANES and calculations in monoclinic Li2FeSiO4 structure for more than one Li extraction. All of these findings reveal some missing links in our fundamental comprehension of these materials and provide ideas for designing/obtaining improved silicate cathode materials.

Thermal-induced reversible ferroelastic phase transition in a new bromethyl-substituted molecular rotor

A new bromethyl-substituted molecular rotor, [Cu(dabcoCH2Br)(H2O)Br3] (dabcoCH2Br+=1-(2-bromethyl)-1,4-diazoniabicyclo[2.2.2]octane cation), which belongs to a family of halomethyl-substituted molecular rotors, was synthesized and structurally characterized. The reversible phase transition at ca. 250 K was well established for this molecular rotor by thermal analyses, variable-temperature X-ray diffraction, and variable temperature dielectric measurements. The order-disorder transformation of the rotator part (dabco moiety) causes ferroelastic phase transition with an Aizu notation of mmmF2/m from high-temperature orthorhombic phase (Pbnm) to low-temperature monoclinic phase (P21/n). More important, in reference to the density functional theory calculations and structural analyses, the key factors to tune the phase transition behaviors are discussed in detail for this family of halomethyl-substituted molecular rotors.

Tuning the monoclinic-to-orthorhombic phase transition temperature of Fe2Mo3O12by substitutional co-incorporation of Zr4+and Mg2+

Abstract

Near‐Zero Thermal Expansion in In(HfMg)0.5Mo3O12

In(HfMg)0.5Mo3O12, which can be considered as a 1:1 mole ratio solid solution of the low‐positive thermal expansion material HfMgMo3O12 and the low‐negative thermal expansion (NTE) material In2Mo3O12 was prepared. From DSC and XRPD results, we show that In(HfMg)0.5Mo3O12 exists in a monoclinic (P21/a) structure at low temperature and undergoes a phase transition at ~425 K to an orthorhombic phase (Pnma), with an associated enthalpy change of 0.89 kJ mol−1. Thermal expansion is large and positive in the low‐temperature monoclinic phase (average αℓ = 16 × 10−6 K−1 and 20 × 10−6 K−1, from dilatometry and XRPD, respectively). Remarkably, this material has a near‐zero thermal expansion (ZTE) coefficient over the temperature range ~500 to ~900 K in the high‐temperature orthorhombic phase, both intrinsically and for the bulk sample. The average linear intrinsic (XRPD) value is αℓ = −0.4 × 10−6 K−1, and the average bulk (dilatometric) value is αℓ = 0.4 × 10−6 K−1 with an uncertainty of ± 0.2 × 10−6 K−1. The slight difference between intrinsic and bulk thermal expansion is attributed to microstructural effects. XRPD results show that the thermal expansion is more isotropic than for the parent compounds HfMgMo3O12 and In2Mo3O12.

Electrical and Dielectric Spectroscopic Characterization of Polycrystalline Dy&lt;sub&gt;2&lt;/sub&gt;Si&lt;sub&gt;2&lt;/sub&gt;O&lt;sub&gt;7&lt;/sub&gt;

The compound Dy2Si2O7 exists in two polymorphs, the low temperature triclinic phase (type B) and a high temperature orthorhombic phase (type E).The dc and ac electrical conductivities of E-Dy2Si2O7 are measured in the temperature range 290-510 K and frequency range 1 kHz to 1 MHz . The dc electrical transport data are analyzed according to Motts variable-range hopping model. The disorder parameter (To) and density of states at fermi level are obtained. The ac conductivity σac (ω) is obtained through the dielectric parameters. The ac conductivity can be expressed as σac (ω) =B ω s , where s is slope and it decreases with increase in temperature. The conduction mechanism in the compound is discussed in low and high temperature regions in the light of theoretical models.