STRUCTURAL LOW-TEMPERATURE PHASE-TRANSITIONS IN YXCA1-XBA2CU4O8 SINGLE-CRYSTALS - RESULT OF THE SPIN GAP OPENING

Temperature dependent measurements of the lattice constants of YxCa1-xBa2Cu4O8 single crystals in the range of 100 - 300 K showed a phase transition near 150 K which is more pronounced in Ca-124 than in unsubstituted crystals. The correlation of this transition with the spin gap opening of 124 below 160 K is discussed. Single crystal structure refinements show an abrupt contraction of the O - Y(Ca) - O sandwich in Ca-124 below 160 K. In addition, a much stronger contraction of the Cu(2) - O(1) apical bondlength is observed in Ca-124 crystals at lower temperatures. This gives for the first time an explanation for the increase of T-C in 124 by Ca-substitution.