From thermodynamics, local spin density approximation Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO 2 stripes we find that Li 2 ZrCuO 4 (Li 2 CuZrO 4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility T and specific heat c p T; H within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals ÿJ 2 =J 1 0:3 is close to the critical value c 1 4 . Comparing with related chain cuprates we explain the rather strong field dependence of c p , the monotonic downshift of the peak of T, and its increase for ! c 0.
The physical properties of perovskites of the type AC 3 B 4 O 12 , whose structure derives from simple perovskites ABO 3 , are reviewed. The A position is subject to strong structural distortions and splits into two new positions A and C. In the structure of AC 3 B 4 O 12 vacancies and any cations with a large radius, irrespective of their charge state, can be present in the icosahedral environment of A: Na + , Cd 2+ , Ca 2+ , Sr 2+ , Y 3+ , Ln 3+ , and Nd 4+ . The C position in the square environment of oxygen can be occupied only by the Jahn-Teller cations Cu 2+ and Mn 3+ . Transition and nontransition metal ions-Mn 3+ , Fe 3+ , Al 3+ , Cr 3+ , Ti 4+ , Mn 4+ , Ge 4+ , Ru 4+ , Ir 4+ , Ta 5+ , Nb 5+ , Ta 5+ , Sb 5+ -can occupy the B position in an octahedral environment. Some members of the family of complex perovskites possess properties which are characteristic for systems with heavy fermions; collinear and noncollinear magnetic structures with high ordering temperatures occur in these materials; tunneling magnetoresistance and high permittivity are observed. The diversity and unique properties make these materials attractive for practical applications.
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