PHYSICAL REVIEW B VOLUME 51, NUMBER 5 1 FEBRUARY 1995-I Magnetic phase transitions and crystal-field splitting in K2UX5 (X =Cl, Br,I) L. Keller, * A. Furrer, P. Fischer, and P. Allenspach Laboratorium fur Neutronenstreuung, Eidgenossische Technische Hochschule Ziirich and Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland K. Kramer and H. U. Gudel Institut fii r Anorganische, Analytische und Physikalische Chemic, Universitat Bern, CH3000'-Bern, Switzerland A. Donni The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-01, Japan T. Suzuki Department of Physics, Faculty of Science, Tohoku University, Sendai 980, Japan (Received 28 July 1994; revised manuscript received 30 September 1994) A consistent physical analysis of both elastic- and inelastic-neutron-scattering experiments as well as of specific-heat measurements in the temperature range from 7 mK to 295 K on polycrystalline samples of the Sf' electron system K2UX5 (X=C1,Br,I) was performed on the basis of a two-dimensional Ising model. The orthorhombic structure of K2PrC15 type was refined in the paramagnetic state and turned out to be built up by well-separated chains of [UX3X4&2] polyhedra. K2UC1, and K~UBr, undergo a magnetic phase transition to a long-range antiferromagnetically ordered state corresponding to the Shubnikov space group Pn'm'a' with k=O at Neel temperatures T&=3.8(1) K and T&=2.8(1) K, re- spectively. K2Ul, orders antiferromagnetically at T&=1.45(3) K in the Shubnikov space group Pnm'a with k=O. The ordering temperatures, the dominant intrachain exchange interactions, as well as the magnitudes of the magnetic moments at saturation [p, „=3. 24(4)ps, 2.31(4)ps, and 1.80(4)ps for X= Cl, Br, and I, respectively] are strongly dependent on the intra- and interchain uranium distances. Specific- heat measurements showed large magnetic contributions at temperatures higher than T~ for all com- pounds K2UX5 (X=C1,Br,I). Analyzed in terms of the anisotropic two-dimensional Ising model, these measurements proved the existence of short-range magnetic ordering along the uranium chains above Tz, followed by the long-range ordered magnetic phase. By means of inelastic neutron scattering we determined the crystal-field splitting of the series K2UX5 (X=C1,Br,I) from well-defined crystal-field peaks. Due to the low local symmetry at the uranium site (sevenfold coordination), the crystal-field level scheme was analyzed introducing geometrical dependences of crystal-field parameters which provide a good description of both crystal-field energy levels and transition probabilities. I. INTRODUCTION The study of magnetic ordering phenomena in uranium (III) halides has for many years been subject of several, mostly bulk magnetic investigations. However, magnetic structure determinations applying neutron-scattering techniques have so far only been carried out for binary halides UX3 such as UC13, ' UBr3, ' and UI3. In Ref. 1 the magnetic properties of UC13 were interpreted on the basis of uranium chains with predominant antiferromag- netic intrachain and weaker interchain interaction. Schmid et al. reported on one-dimensional spin-wave ex- citations in UX3 (X=Cl, Br) observed by means of neu- tron spectroscopic investigations. The aim of this work is the investigation of magnetic behavior of uranium chain systems. Compounds with well-separated chains may act as model systems for the investigation of intra- and interchain interactions and therefore for the investigation of the short- and long- range magnetic ordering phenomena in predominantly low-dimensional systems. Ternary halides K2UX5 (X=C1,Br, I) of uranium are nonmetallic 5f electron systems with a 19/2 ground state of the U + ions. These compounds crystallize in the orthorhombic KzPrC15-type structure corresponding to space group Pnma. ' The structure may be thought as a stacking of well-separated chains of [UX3X4&2] polyhe- dra which are connected over rather large potassium con- tacts. Bulk magnetic measurements of the chlorides A &UC15 ( A = K, NH~, Rb) (Ref. 7) give evidence for anti- ferromagnetic ordering. However, a first attempt to determine the magnetic structure of K2UCls (Ref. 8) was not successful because of limited sample quality. Recent- ly, the magnetic ordering of KzUBr5 was investigated. Neutron-di6raction experiments reveal a long-range mag- netically ordered phase below T&=2. 8(1) K, whereas measurements of the bulk magnetic susceptibility give evidence for low-dimensional magnetic ordering at tem- peratures higher than T&. Here we report on a consistent physical interpretation 0163-1829/95/51(5)/2881(10)/$06. 00 51 2881 1995 The American Physical Society