Thermodynamic investigation of phase formation processes in the systems LnSe2-x-LnSe1.5 (Ln = La, Ce, Pr, Nd, Sm, Gd)

L. Zelenina1, T. Chusova1 and I. Vasilyeva1

1Nikolaev Institute of Inorganic Chemistry, Novosibirsk, Russia

Keywords: rare-earth polyselenides, static method, phase diagram
property: vapour pressure, standard heat of formation, absolute entropy
material: low-dimensional compounds

Polyselenides of rare-earth elements are interest of solid-state physics in studying phenomena of anionic cluster formation, which   are possible due to the ability of selenium to form partial and extended Se-Se bonding in solids. For investigation physical properties of such low-dimensional compounds the single crystals of intermediate polyselenides from the LnSe2-xLnSe1.5 systems are required. The preparation of such crystals is a difficult problem without knowledge of the thermodynamic parameters (pSe(T, x)), which define the condition of crystals growth.  The aim of this work is to obtain reliable data on compositions  of intermediate phases and to describe their growth conditions. The vapour pressure measurement of the investigated compounds has been realized by the static method. This method is suitable for the investigation of the equilibrium states in systems, even if the processes go very slowly. High-quality crystals of GdSe1.875, SmSe1.9 and LnSe2.00 (Ln = La, Ce, Pr, Nd) were investigated.  The measurements have been realized in wide intervals of pressure (1≤ р, Torr≤760), temperature (713≤T, K≤1395) and composition (1.5≤ x, f. u. ≤2.0) with limiting errors 0.5 Torr, 0.5 K and 0.01 formula units, accordingly.. The detailed study revealed that the phase regions in composition intervals studied consist of discrete intermediate phases [1] which may be described by general formula LnnSe2n-1 (n = 4, 5, 7, 8, 10, 20). Most of these phases were detected for the first time. Our investigation does not confirm the existence of homogeneity regions, which were described earlier in literature. The thermodynamic characteristics for dissociation processes were obtained and the standard heats of formation and the absolute entropies of the investigated compounds were calculated. As a result of this study, the thermodynamic parameters pSe and T, which define the fields of stability and the growth conditions of homogeneous La, Ce, Pr, Nd, Sm and Gd polyselenides phases of a specified composition were determined.

  1. L. Zelenina, T. Chusova, I. Vasilyeva J. Therm. Anal. and Calorimetry, 101, 59 (2010)

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