• P S R Krishna

Articles written in Pramana – Journal of Physics

• Temperature effects on the structure of liquid D-methanol through neutron diffraction

The study of changes in the structure (H-bonded) of liquid alcohols at elevated temperatures is rare probably due to low flash points of these liquids. An indigenously devised special quartz cell is now used to carry out the structural studies of these liquids at elevated temperatures through neutron diffraction. Here, the liquid consists of deuterated methanol and neutron data was collected on the high-Q diffractometer at Dhruva, BARC. The corrected data at elevated temperatures (BP (boiling point) and double the BP) show that there is a large change in the H-bonded structure of this liquid. The pre-peak or hump, known to be signature of H-bonded clusters appears to be present at all the three temperatures studied. In the low-Q (scattering vector) data Ornstein Zernike (OZ) behaviour is also observed. It is, however, seen that the intramolecular structure does not change very much at higher temperatures. A detailed model analysis is in progress and would be reported later.

• Neutron diffraction studies on La2−xDyxCa2xBa2Cu4+2xOz superconductors

Structural studies on Dy-substituted La-2125 type superconductors have been carried out by neutron diffraction experiments at room temperature using a monochromatic neutron beam of wavelength (λ) = 1.249 Å. A series of samples with La2-xDyxCa2xBa2Cu4+2xOz stoichiometric composition, forx = 0.1–0.5, have been studied for their structural properties. A tetragonal Y-123 unit cell was taken as the starting model for the Rietveld analysis. All the samples fit into the starting model, with no structural transition taking place with increasing dopant concentration. The results of Rietveld analysis and structural properties will be discussed in detail.

• Structure and magnetic properties of colossal magnetoresistance compound Tb0.5Sr0.5CoO3

The structure and the magnetic properties of the doped rare earth cobaltite systems are of recent interest owing to the CMR phenomenon that occur in them. In this paper, we investigate the structure and magnetic properties of Tb0.5Sr0.5CoO3 solid solution, for the first time, using neutron powder diffraction technique. The sample Tb0.5Sr0.5CoO3 is found to crystallize in orthorhombic (Pbnm) symmetry. The unit cell volume and Co—O bond length reduce with temperature. The calculatedeg bandwidth obtained from structural parameters turns out to be 0.989 eV. Low temperature neutron diffraction profiles exhibit a magnetic contribution to the fundamental Bragg peaks indicating a ferromagnetic ordering belowTc. The results are compared with Co—O—Co bond angles and Co—O bond length of La0.5Sr0.5CoO3, highlighting the ionic size effects on substitution of Tb ion for La in the compound.

• Low temperature magnetic structure of MnSe

In this paper we report low temperature neutron diffraction studies on MnSe in order to understand the anomalous behaviour of their magnetic and transport properties. Our study indicates that at low temperatures MnSe has two coexisting crystal structures, high temperature NaCl and hexagonal NiAs. NiAs phase appears below 266 K and is antiferromagnetically ordered at all temperatures while the NaCl phase orders antiferromagnetically at 130 K.

The HgO-added YBa2Cu3O7−δ (YBCO) superconductor has been studied for its structural and superconducting properties. Polycrystalline YBCO samples were synthesized through solid-state reaction method by adding HgO in different concentrations without using oxygen annealing. All the samples showed a sharp superconducting transition temperature around 90 K. The X-ray diffraction patterns of all the samples revealed monophasic Y-123 nature. The structural studies were carried out by neutron scattering and Rietveld analysis. The neutron scattering revealed that Hg is not incorporated in the Y-123 system and has shown optimum oxygen concentration. The significant role played by the HgO is to provide oxygen ambient through its decomposition, thus changing the oxygen balance in favour of high Cu-valence state.

• Neutron scattering study of combustion-synthesized Ce1−xCuxO2−x

Powder neutron diffraction and Hi-Q neutron diffraction data have been recorded and analysed in order to obtain the local and long range order of Cu in Cudoped CeO2 with three doping levels of Cu. Rietveld method and MCGR techniques of data analysis for the two types of data reveal that the Cu ion is in the 2+ oxidation state and has a vacancy in its first coordination shell. These deductions from the data analysis fit well with the mechanism of catalysis we propose.

• Low-temperature neutron diffraction study of La0.95Nd0.05CrO3

We have synthesized polycrystalline La0.95Nd0.05CrO3 sample by doping the La-site of LaCrO3 with Nd and its magnetic properties have been studied using DC magnetization and neutron diffraction techniques. DC magnetization study shows a paramagnetic to a weak ferromagnetic-like transition at ∼295 K followed by signatures of a spin reorientation phenomenon at 233 and 166 K and, finally a transition to an antiferromagnetic-like phase at ∼21 K. Low-temperature neutron diffraction measurements confirm a weak ferrimagnetic ordering of Cr3+ moments at all temperatures below 295 K.

• Molecular conformation and structural correlations of liquid D-1-propanol through neutron diffraction

An analysis of neutron diffraction data of liquid deuterated 1-propanol at room temperature to extract its molecular conformation is presented. Being a big molecule with twelve atomic sites, the analysis is tricky and needs careful consideration. The resulting molecular parameters are compared with electron diffraction (gas phase), X-ray diffraction (liquid phase) and MD simulation results. Information about the hydrogen-bonded intermolecular structure in liquid is extracted and nature of the probable molecular association suggested.

• Structural behaviour of AgNO3 at low temperatures by neutron diffraction

Structural behaviour of silver nitrate (AgNO3) at low temperatures has been investigated by neutron powder diffraction and differential scanning calorimetry (DSC). Analysis showed abnormal changes in the rotations of nitrate (NO3) anions and thermal displacement parameters of the atoms near 220 K and 125 K. However, the basic lattice is compatible with the orthorhombic symmetry (space group Pbca) till 12 K. The fine, small-scale structural anomalies probably originate from freezing of reorientation of NO3 ions from high-temperature disordered phase.

• Cation disorder and structural studies on Bi$_{4−x}$Nd$_{x}$Ti3O12 $(0.0 \leq x \leq 2.0)$

Here we report the results of combined powder X-ray and neutron diffraction studies of Bi$_{4−x}$Nd$_{x}$Ti3O12 $(0.0 \leq x \leq 2.0)$ compositions. The parent Bi4Ti3O12 has an orthorhombic lattice (space group: B2cb) with unit cell parameters $a$ = 5.4432(5) Å, $b$ = 5.4099(5) Å and $c$ = 32.821(2) Å, and $V$ = 966.5(1) Å3. This orthorhombic lattice is retained in all the studied compositions. The unit cell parameters gradually decrease with Nd 3+ ion concentration with a discontinuity at $x = 0.75$. Orthorhombicity of the lattice decreases with increase in Nd3+ content in the lattice. The orthorhombic unit cell parameters for a representative Bi2Nd2Ti3O12 composition are: $a$ = 5.3834(9), $b$ = 5.3846(9) and $c$ = 32.784(1) Å. The observed orthorhombic distortion at $x$ = 2.0 is very small and thus the crystal structure apparently has a pseudo-tetragonal lattice. In addition, Nd3+ preferentially substitutes in the perovskite slab of the Aurivillius structure. The fraction of Nd3+ in the fluorite slab increases with increase in Nd3+ contents.

• Neutron diffraction studies on Ca$_{1-x}$Ba$_{x}$Zr4P6O24 solid solutions

Herein we report the results of detailed crystallographic studies of Ca$-[1-x}$Ba$_{x}$Zr4P6O24 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 Ba2+ concentration contrast to an increasing trend in 𝑐 parameter.

• Molecular conformation and liquid structure of 2-propanol through neutron diffraction

The neutron diffraction data analysis of deuterated liquid 2-propanol at room temperature to define its molecular conformation is presented. 2-Propanol being a large molecule with twelve atomic sites, the conformation analysis is tricky and an improved method of data analysis is given. The intermolecular structural correlations, i.e., hydrogen-bonded liquid structure, can be modelled accurately to extract the nature of the average hydrogen-bonded molecular association in liquid state at room temperature. Like other alcohols these are mostly hexamer ring chain (HRC) clusters. The cluster analysis of recent X-ray data available in the literature also support the same liquid structure.

• # Pramana – Journal of Physics

Volume 96, 2022
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• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019