High-pressure study of the Raman modes in YBa2(Cu0.96Ni0.04)(4)O-8

We present a study of the Raman phonons in YBa2(Cu0.96Ni0.04)(4)O-8 under h ydrostatic pressure in the superconducting phase. A comparison with our ear lier work on the undoped YBa2Cu4O8 shows that the pressure coefficients of two vibrational modes involving oxygen atoms differ significantly from thos e of the corresponding modes in the undoped material. These are the O(1) ch ain mode which shifts 33% faster and the O(2)-O(3) in-phase mode which shif ts 23% slower than the undoped counterparts. The other Raman modes in the N i-doped sample shift in a manner similar to the undoped material. The obser ved behavior of the O(1) chain and O(2)-O(3) in-phase modes in YBa2(Cu0.96N i0.04)(4)O-8 under pressure and;the softening of the Cu(1) A(g) mode freque ncy with increasing Ni doping suggest that the Ni atoms substitute for the Cu atoms in the chain, which in rum decreases the compressibility in the vi cinity of the Cu(Ni) chain atom. [S0163-1829(99)14429-1].