Effect of the lanthanide ion on the structure and low-temperature phase transitions in RbL(SO4)2.4H2O (L identical to La-Er) crystals

Crystallographic unit cell parameters of RbL(SO 4 ) 2 .4H 2 O (L identical to La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) are 296 K and specific heat capacity (C p ) data on RbL(SO 4 ) 2 .4H 2 O (L identical to Pr, Nd, Sm, Eu, Gd, Tb, Dy, and Er) for 110<T<305 K are presented. The RbL(SO 4 ) 2 .4H 2 O lattices form an isostructural monoclinic series and have the layered structure of RbSm(SO 4 ) 2 .4H 2 O. The unit cell parameters a, b, c and beta reflect an unusual trend with lanthanide size, unlike other isostructural lanthanide series where cell parameters are monotonically related to the lanthanide contraction. The variation in the cell parameters was schematically used to deduce the trend in the buckling of lanthanide-sulphate layers in RbL(SO 4 ) 2 .4H 2 O, and it is argued that the observed nonmonotonic behaviour in cell parameters is a consequence of this buckling. The C p measurements indicate that RbL(SO 4 ) 2 .4H 2 O (L identical to Pr, Nd, Eu, Gd, Tb and Dy) lattices undergo a lambda transition. Additional structural transitions are observed in RbPr(SO 4 ) 2 .4H 2 O, RbNd(SO 4 ) 2 .4H 2 O, and RbDy(SO 4 ) 2 .4H 2 O, while no transitions occur in RbEr(SO 4 ) 2 .4H 2 O for 110<T<305 K. The lattice stability model, inclusive of the buckling effects, successfully maps the observed trend in T lambda as well as in the order-disorder transition T 2 in isostructural NH 4 L(SO 4 ) 2 .4H 2 O (L identical to La-Tb) lattices.