Normal-state specific heat of YBaCuO oxides

Oxygen reordering near room temperature inYBa2Cu3O6+x<

“…Second, if the structure of the material also is inhomogeneous, for example, containing twins, grain boundaries, a stress field will be established in various areas of the phase transformation, and possibly will obstruct or enhance the occurrence of the phase transformation. Despite many reports on the SIOR phase transformation, [34][35][36][37][38][39][40][41][42] the values of changes in specific heat, the thermal-expansion coefficients, and the elastic compliances during phase transformation are seldom reported. The exceptions are Nagel et al's 34 measurements of the thermal-expansion coefficients in untwinned single crystals, and the work of Cankurtaran and his co-workers, 37,38 who assessed the changes in elastic stiffness in an isotropic polycrystal sample.…”

“…[4][5][6][7][8][9] Various theoretical models [10][11][12][13][14][15][16][17][18][19][20][21][22][23] and experimental techniques [23][24][25][26][27][28][29] have been used to study normal oxygen ordering in YBCO, from the disordering to the ordering critical temperature T c about 1000 K. In substoichiometric YBCO oxides (x < 1), other oxygen ordering transformations, such as ortho I to ortho II or ortho III, and so on also were demonstrated experimentally 23,30,31 and analyzed theoretically. 11,[13][14][15][16]18,22,32,33 However, another phase transformation, occurring about 220-280 K, which relates to the discontinuous change of material's lattice constants, 34 specific heat, 35,36 elastic stiffnesses, 37-40 electrical resistivity, 41 and dielectric constant, 42 has only been revealed experimentally. Although most experimenters deemed that this phase transformation is related to the reordering of oxygen atoms, ...…”

“…11,[13][14][15][16]18,22,32,33 However, another phase transformation, occurring about 220-280 K, which relates to the discontinuous change of material's lattice constants, 34 specific heat, 35,36 elastic stiffnesses, 37-40 electrical resistivity, 41 and dielectric constant, 42 has only been revealed experimentally. Although most experimenters deemed that this phase transformation is related to the reordering of oxygen atoms, [34][35][36][37][38][39][40][41] it has no theoretical explanation until now. In this paper, we suggest a thermodynamic framework and lattice distortion mechanism to explain this oxygen-reordering process and then discuss some thermodynamic response functions on either side of the critical temperature of the phase transformation.…”