S. Achary - Academia.edu (original) (raw)

Papers by S. Achary

Herein we report preparation, thermal stability and magnetic properties of a complex phosphate of... more Herein we report preparation, thermal stability and magnetic properties of a complex phosphate of manganese (Mn5(HPO4)2(PO4)2 4H2O) from powder XRD, thermogravimetric and magnetization studies. The structure of the composition has cluster of five Mn2+ions each having octahedral coordination. The composition is stable up to 300°C and then it decomposes to Mn2P2O7 and Mn3(PO4)2. The FC and ZFC magnetic measurements indicate a ferromagnetic like sharp transition around 6 K. Below transition temperature, the field dependent magnetization indicates ferromagnetic like hysteresis loop and at higher field signature of antferromagnetic interactions is observed.

Journal of Materials Science, 2002

... undergo different ori-entations [3]. The negative thermal expansion behavior is also reported... more ... undergo different ori-entations [3]. The negative thermal expansion behavior is also reported in silica [4], ice [5] and zeolites [6]. Negative ... However, any reversible pressure induced transition could not be ruled out. ... The amorphization of ZrW2O8 is also an irreversible transition. ...

Materials Research Bulletin, 2003

Series of the mixed fluorides with the general composition M 1Àx Nd x F 2þx (0:00 x 1:00; M ¼ Sr ... more Series of the mixed fluorides with the general composition M 1Àx Nd x F 2þx (0:00 x 1:00; M ¼ Sr 2þ and Ca 2þ) were prepared by a vacuum heat treatment of the appropriate mixtures of MF 2 (M ¼ Ca and Sr) and NdF 3. The products obtained were analyzed by powder X-ray diffraction, which revealed the phase relations in these systems. At the MF 2-rich compositions a fluorite-type solid solution was observed in both systems. The typical solid solution limits of NdF 3 in the SrF 2 and CaF 2 lattice are about 40 and 45 mol%, respectively. The incorporation of NdF 3 in the fluorite lattice of CaF 2 causes an expansion of the unit cell volume. The unit cell volume of the fluorite lattice in Sr 1Àx Nd x F 2þx system surprisingly remains constant throughout the solid solution range. The NdF 3 (tysonite) type phase separates out beyond the solid solution limit, in both systems. Also, it was observed that about 15 and <10 mol% of CaF 2 and SrF 2 , respectively, could be retained in the NdF 3 , maintaining the tysonite lattice. No fluorite or tysonite-related ordered phases were observed in these systems.

Arxiv preprint arXiv: …, 2011

X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to ... more X-ray diffraction and Raman-scattering measurements on cerium vanadate have been performed up to 12 and 16 GPa, respectively. Experiments reveal that at 5.3 GPa the onset of a pressure-induced irreversible phase transition from the zircon to the monazite structure. Beyond this pressure, diffraction peaks and Raman-active modes of the monazite phase are measured. The zircon to monazite transition in CeVO 4 is distinctive among the other rare-earth orthovanadates. We also observed softening of external translational T(E g) and internal ν 2 (B 2g) bending modes. We attributed it to mechanical instabilities of zircon phase against the pressure-induced distortion. We additionally report lattice-dynamical and total-energy calculations which are in agreement with the experimental results. Finally, the effect of non-hydrostatic stresses on the structural sequence is studied and the equations of state of different phases are reported.

The Journal of Physical Chemistry C, 2010

Solid State Communications, 2014

We performed in situ X-ray diffraction experiments on Bi 14 CrO 24 under pressure up to 17.4 GPa.... more We performed in situ X-ray diffraction experiments on Bi 14 CrO 24 under pressure up to 17.4 GPa. We discovered two reversible phase transitions that take place at 7.3 and 12.1 GPa. The first transition is isostructural and the second-transition involves a tetragonal-monoclinic symmetry reduction. Both transitions involve a small volume collapse indicating that they have a first-order character. For the three observed phases we determined a P-V equation of state. All of them are highly compressible having bulk moduli that range from 64 to 70 GPa. We also determined the axial compressibilities for difference phases, being the response of the structures to pressure anisotropic. This behavior is more notorious in the high-pressure monoclinic phase. Finally, the observed structures are related and compared with the two previously known polymorphs of Bi 14 CrO 24 .

$Research paper thumbnail of Neutron diffraction studies on Ca1−x Ba x Zr4P6O24 solid solutions$

Pramana, 2008

Herein we report the results of detailed crystallographic studies of Ca1−xBaxZr4P6O24 composition... more Herein we report the results of detailed crystallographic studies of Ca1−xBaxZr4P6O24 compositions from combined Rietveld refinements of powder X-ray and neutron diffraction data. All the studied compositions crystallize in rhombohedral lattice (space group R-3 No. 148). A continuous solid solution is concluded from the systematic variation of unit cell parameters. The variation of unit cell parameters with the composition indicates decreasing trend in a parameter with increasing Ba 2+ concentration contrast to an increasing trend in c parameter.

Journal of the American Ceramic Society, 2013

ABSTRACT A series of compositions with general stoichiometry Ca1−xZr1−xNd2xTi2O7 has been prepare... more ABSTRACT A series of compositions with general stoichiometry Ca1−xZr1−xNd2xTi2O7 has been prepared by high-temperature solid-state reaction of component oxides and characterized by powder X-ray diffraction and electron probe for microanalyses (EPMA). The phase fields in CaZrTi2O7–Nd2Ti2O7 system and distribution of ions in different phases have been determined. Four different phase fields, namely monoclinic zirconolite, cubic perovskite, cubic pyrochlore, and monoclinic Nd2Ti2O7 structure types are observed in this system. The 4M-polytype of zirconolite structure is stabilized by substitution of Nd3+ ion. The addition of Nd3+ ions form a cubic perovskite structure-type phase and thus observed in all the compositions with 0.05 ≤ x ≤ 0.80. Cubic pyrochlore structure-type phase is observed as a coexisting phase in the nominal composition with 0.20 ≤ x ≤ 0.90. Only a subtle amounts of Ca2+ and Zr4+ are incorporated into the perovskite-type Nd2Ti2O7 structure. EPMA analyses on different coexisting phases revealed that the pyrochlore and perovskite phases have Nd3+-rich compositions.

Journal of Solid State Chemistry, 2005

ABSTRACT We report the results of the high-pressure angle dispersive powder X-ray diffraction mea... more ABSTRACT We report the results of the high-pressure angle dispersive powder X-ray diffraction measurements obtained up to 31 GPa on the monoclinic and the tetragonal phases of NbOPO4. The tetrgonal phase was obtained by quenching the monoclinic phase from 5 GPa and 833 K. The lattice parameters of the monoclinic phase show an anomaly at 0.6 GPa, but the unit cell volume follow an almost linear compressional behavior. The bulk modulus B0 and its pressure derivative are estimated to be 66(3) GPa and 1.6(8), respectively, for the monoclinic phase. The irreversible pressure induced amorphization observed at ambient temperature in the monoclinic phase is due to the kinetic hindrance to bond reconstruction that is required for transformation form the monoclinic to the tetragonal phase. In the case of tetragonal NbOPO4, the compression is found to be anisotropic.

$Research paper thumbnail of Investigation of the phase stability of LuVO4at high pressure using powder x-ray diffraction measurements and lattice dynamical calculations$

Journal of Physics: Condensed Matter, 2008

... Results of investigation on the structural stability of LuVO4 up 33 GPa are reported. The res... more ... Results of investigation on the structural stability of LuVO4 up 33 GPa are reported. The results support the existence of a zircon to scheelite type first-order phase transition in zircon-structured LuVO4. The lattice dynamical calculations support the experimental observations. ...

Journal of Physics: Condensed Matter, 2006

The high-pressure behaviour of scheelite-structured ZrGeO 4 and HfGeO 4 has been investigated wit... more The high-pressure behaviour of scheelite-structured ZrGeO 4 and HfGeO 4 has been investigated with the help of angle-dispersive powder x-ray diffraction measurements. Our results show that these compounds do not undergo any phase transition up to the pressures of 20.7 and 19.0 GPa, respectively. The isothermal bulk modulus and its pressure derivatives are found to be 238 GPa and 4.5 for ZrGeO 4 , and 242 GPa and 4.8 for HfGeO 4 , implying that these germanates are highly incompressible.

Journal of Physics and Chemistry of Solids, 2008

The high pressure and high temperature structural investigations on a and b phases of HfMo 2 O 8 ... more The high pressure and high temperature structural investigations on a and b phases of HfMo 2 O 8 are reported. a-HfMo 2 O 8 transforms to a monoclinic (d) phase, followed by a second transition to a triclinic () phase at 1.30(5) and 2.70(5) GPa, respectively. a-HfMo 2 O 8 undergoes pressure induced amorphization above 24 GPa due to kinetically hindered phase transition to its high density polymorph b phase. The b-HfMo 2 O 8 also undergoes a complete pressure induced amorphization above 35 GPa due to the kinetic hinderance to decomposition. When annealed at ambient pressure, b-HfMo 2 O 8 reconverts to the trigonal a phase above 700 1C.

Journal of Physics and Chemistry of Solids, 2011

The results of high-pressure angle dispersive X-ray diffraction measurements up to 34.3 GPa on th... more The results of high-pressure angle dispersive X-ray diffraction measurements up to 34.3 GPa on the double perovskite Ba 2 MgWO 6 are presented. The ambient rock salt phase (SG: Fm-3m) is found to be stable up to the highest pressure of the present measurements. The third order Birch-Murnaghan equation of state when fitted to pressure-volume data, yielded a zero pressure bulk modulus (B 0),and its first and second pressure derivatives as 137.0(81) GPa, and 3.9(5) and À 0.03 GPa À 1 , respectively.

Journal of Physics: Conference Series, 2012

Herein we report the results of high pressure diffraction studies of zircon type ThGeO4. ThGeO4 e... more Herein we report the results of high pressure diffraction studies of zircon type ThGeO4. ThGeO4 exhibits anisotropic compressibility with the average compressibility along a-axis (20.8 × 10−4/GPa) larger than that along c-axis (9.98 × 10−4/GPa). Fitting the pressure dependence unit cell volume to 3rd order Birch-Murnaghan equation of states, the zero pressure bulk modulus (Ko) and volume (Vo) of 166(5)

Journal of Physics: Conference Series, 2007

Isotropic negative thermal expansion (NTE) is found in cubic AX2O8(A = Zr, Hf: X=W, Mo) up to hig... more Isotropic negative thermal expansion (NTE) is found in cubic AX2O8(A = Zr, Hf: X=W, Mo) up to high temperatures (1050 K). Anharmonicity of low energy phonon modes plays an important role in leading to the NTE behaviour. Earlier we verified our predictions of large phonon softening for low energy phonons (below 8 meV) through high-pressure inelastic neutron scattering measurements on powder samples at ILL, France. Now we have measured the phonon dispersion relation from a single crystal of ZrW2O8. The measurements are useful to verify our prediction of highly anharmonic nature of specific phonon branches, in particular the transverse acoustic branch, and other branches up to 10 meV. These modes below 10 meV mainly contribute to the NTE in ZrW2O8.

Journal of Materials Research, 2010

A new elpasolite-type (NH4,K)3VO2F4 compound was prepared and characterized by x-ray diffraction,... more A new elpasolite-type (NH4,K)3VO2F4 compound was prepared and characterized by x-ray diffraction, differential scanning calorimeter (DSC), impedance analysis, and electrical polarization measurements. It crystallizes in an orthorhombic lattice with unit-cell parameters: a = 8.9584(4), b = 18.6910(14), c = 6.2174(4) Å, V = 1041.04(11) Å3, Z = 6. NH4+, and K+ ions are distributed statistically over crystallographically four equivalent sites. There are two distinguishable vanadium atoms forming cis- and trans-VO2F4 octahedra present in the unit cell. High-temperature studies by DSC and in situ x-ray diffraction revealed a first-order structural transformation from orthorhombic to cubic lattice around 343 K. Impedance measurements show two different kinds of conductivity behaviors for the two phases. In orthorhombic phase a significant conductivity resulting from involvement of protonic species is observed. In the orthorhombic phase, a clear ferroelectric hysteresis loop is observed.

Journal of Alloys and Compounds, 2005

The high-temperature behavior of Ba 2 MgWO 6 and Sr 2 MgWO 6 double perovskite compounds is studi... more The high-temperature behavior of Ba 2 MgWO 6 and Sr 2 MgWO 6 double perovskite compounds is studied by high-temperature XRD (HT-XRD) and dilatometer. At ambient temperature, Ba 2 MgWO 6 crystallizes in cubic symmetry with space group Fm-3m (a = 8.1345(2)Å and V = 538.26(2)Å 3 , Z = 4). The later compound crystallizes in tetragonal symmetry with space group I4/m (a = 5.5876(1) and c = 7.9490(3)Å; V = 248.18(1)Å 3 , Z = 2). HT-XRD studies on these compounds were carried out in the temperature range of 25-1200 • C at each 100 • C interval in the static air atmosphere. Ba 2 MgWO 6 is found to expand linearly in the studied temperature range. At and above 300 • C, a cubic elpasolite-type phase is observed for Sr 2 MgWO 6. Within the tetragonal lattice, this compound shows anisotropic expansion. In the temperature range of 25-1200 • C, the typical volume thermal expansion coefficients of Ba 2 MgWO 6 and Sr 2 MgWO 6 are 35.6 × 10 −6 and 39.6 × 10 −6 / • C, respectively. The details of the phase transition and thermal expansion are explained in this manuscript.

Inorganic Chemistry, 2010

A series of samples with composition Gd 2-y Ce y Zr 2 O 7 (0.0 e y e 2.0) were prepared by the ge... more A series of samples with composition Gd 2-y Ce y Zr 2 O 7 (0.0 e y e 2.0) were prepared by the gel combustion method followed by high-temperature reduction. The details of the structural variations as a function of the composition, temperature, and oxygen stoichiometry have been investigated by X-ray diffraction (XRD), hightemperature XRD (HT-XRD), and thermogravimetry. A complete solubility of Gd 3þ in Ce 2 Zr 2 O 7 and Ce 2 Zr 2 O 8 could be achieved by this adaptive preparative method. Analysis of the XRD data revealed a sequential variation of the structural features with oxygen stoichiometry as well as Gd 3þ contents in these compositions. The variation in the unit cell parameter along the compositions has a strong influence on the oxygen uptake behavior in the Gd 2-y Ce y Zr 2 O 7 system, as observed from the thermogravimetric and HT-XRD studies. The preparation and stability of various metastable phases in Gd-Ce-Zr-O have been addressed in detail. The details of the study will be useful for the design and application of a potential redox catalyst and an oxygen storage capacitor.

Chemistry of Materials, 2009

In this paper, we report the structural and compostional variation of Ce 2 Zr 2 O 7 pyrochlore un... more In this paper, we report the structural and compostional variation of Ce 2 Zr 2 O 7 pyrochlore under a controlled oxidation/reduction process. On oxidation Ce 2 Zr 2 O 7 transforms to Ce 2 Zr 2 O 8 via an intermediate lattice at Ce 2 Zr 2 O 7.5. Crystal structures of the Ce 2 Zr 2 O 7 , Ce 2 Zr 2 O 7.5 , and Ce 2 Zr 2 O 8 have been determined accurately from the neutron diffraction data of polycrystalline samples. The retention of cubic pyrochlore-type arrangements is observed even up to the fully oxidized Ce 2 Zr 2 O 8 composition. The unit cell parameters systematically decreased from Ce 2 Zr 2 O 7 (10.6924(3) Å) to Ce 2 Zr 2 O 8 (10.5443(2) Å). All three compositions retain the original pyrochlore-type cation ordering without any intermixing Ce and Zr atoms. All three structures can be explained as cubically coordinated metal atoms, which share the edge of their AO 8 polyhedra similar to that in the fluorite lattice. The transformation from Ce 2 Zr 2 O 7 to Ce 2 Zr 2 O 8 systematically lowers the symmetry from Fd3m to P2 1 3. A complete oxidation of Ce 3þ to Ce 4þ is observed, in contrast to the kinetically and sterically hindered oxidation proposed earlier. The deep crystallographic insights associated with the oxygen intercalation and deintercalation process in Ce 2 O 3 /CeO 2-ZrO 2 will have immense importance in the development and understanding of new oxygen storage capacitors.