STRUCTURAL INSTABILITIES IN LANTHANUM CUPRATE SUPERCONDUCTORS

Subtle but well characterized structural phase transformations occur a t low temperatures in some lanthanum cuprate superconductors. These tr ansformations, which involve collective tilting of the CuO6 octahedra, have a strong influence on superconducting and normal state propertie s. Experiments suggest that for a given hole doping the following hier archy exists for superconductivity in these materials: T(C) (HTT)> T(C )(LTO)> T(C)(LTT), where HTT represents the untilled structure and LTO and LTT are the two limiting low temperature tilted structures. An ad ditional very strong suppression of T(C), suggestive of a competing el ectronic groundstate, occurs in the LTT phase for a hole to Cu ratio o f 1/8. Neither of these effects are understood at present. This paper reviews both the evidence upon which these conclusions are based and s ome outstanding unanswered questions.