Valence instabilities of Tm in its compounds and solid solutions

— TmSe exhibits both non integral valence and antiferromagnetic order in contrast to most other intermediate valence systems which are non magnetic at low temperatures. We report on measurements of lattice constant, magnetic susceptibility and resistivity made on single crystals of Tm^Se over its range of solid solubility, 0.79 ^ x ;g 1. We find direct correlations among the physical properties, valence and concentration. The fraction of divalent character in TmxSe increases with x, the most Tm deficient sample being essentially trivalent. Alloys of TmSe with YSe have also been prepared as single crystals. Lattice constants and susceptibilities indicate that these materials have non integral valence over the entire system. With increasing Tm concentration a metallic Kondo resistivity changes smoothly towards the high resistivity characteristic of stoichiometric TmSe. Although we find the properties of Tm and Tm configurations in magnetic and transport measurements, since these results are observed in both stoichiometric TmSe and in the dilute single impurity limit, we conclude that all TmSe sites are equivalent. The traditional and highly successful view of rare data [3] (SmS under pressure, Sm^^YjS, SmB6) earths is that for each ion there is an integral number show a single line corresponding to a single non inteof 4f electrons in a single configuration given by gral configuration. The clearest indication of the Hund's rule. The / multiplets are split, in solids, by existence of a new state is the observation of a temthe crystalline electric field. In recent years it has perature independent susceptibility, at low temperabecome apparent that under certain conditions two tures, where a Curie divergence is normally expected. electronic configurations may be nearly degenerate For example, although SmS under pressure is approand the resulting ground state has a non integral ximately 2/3 Sm and 1/3 Sm , no Curie moment number of 4f electrons. A good example of this non associated with Sm is observed [1]. integral valence state is SmS under pressure in which Unstable valence has been associated with Ce, the 4f -4f 5d configurations coexist [1]. Several Sm, Eu, Tm and Yb in various compounds [4]. It is of fundamental questions arise. How does one describe interest to determine the specific conditions required the coexistence of two configurations with highly for the stabilization of the non integral valence state correlated localized f electrons and delocalized d or or the coexistence of two configurations. The factors s electrons ? If one uses the Kondo approach, how involved may include crystal structure, chemical can it be applied to concentrated systems ? What are constituents, band structure, pressure, stoichiometry, the effects of other types of localization such as ranrare earth dilution, and local environmental effects. dom potentials or magnetic polarons ? This paper explores the instability of the Tm valence One of the most interesting experimental questions under a variety of conditions and in the context of the is whether the physical properties are those of the above considerations. individual configurations, their average, orarecompleThe lattice constant is a good indicator of valence tely different. In Sm1_,cYxS XPS [2] shows both the in rare earth compounds and alloys. In the specific f and f d configurations. However, the Mossbauer case of Tm chalcogenides, Iandelli [5] determined, on Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19795108 VALENCE INSTABILITIES OF Tm IN ITS COMPOUNDS AND SOLID SOLUTIONS C5-315 the basis of lattice constant and susceptibility measurements, that Tm was trivalent in TmS and TmSe and divalent in TmTe. These observations were made on powders synthesized at relatively low temperatures (1 400 OC). Subsequently, in a study on single crystals grown at high temperature, Bucher et al. [6] confirmed that the Tm ion was trivalent in TmS, divalent in TmTe but found a range of intermediate lattice constants in TmSe, 5.64 a, 5.71 A, which implied a range of valence extending from Tm3+. In general agreement with the lattice constant results, the high temperature susceptibility obtained by Bucher et al. [6] for TmS and TmTe approach the values for Tm3+ and Tm2+ respectively, while that for TmSe is intermediate. For temperatures which are high with respect to crystal field and any other interaction energies, the susceptibility may be used to determine average valence. Two configurations may coexist as a heterogeneous mixture of integral valence ions or in a homogeneous non integral valence phase. In many systems the low temperature susceptibility clearly distinguishes these cases. For the heterogeneous case, one or the other configurations has a magnetic moment at low temperature and will exhibit a Curie type susceptibility or magnetic order. If neither a Curie susceptibility nor magnetic order is observed, the system is in the homogeneous intermediate valence state. A third possibility is a combination of integral and non integral valence ions. TmSe is of particular importance because it orders antiferromagnetically at low temperatures but has intermediate values of lattice constant and susceptibility at high temperatures. The Tm2+(4f13) ion has a 'F,,, configuration which remains magnetic even in the crystal field. The Tm3+(4f12)3H, configuration is a T, singlet in the cubic field. However, it can develop an induced moment when the exchange interaction is comparable to the crystal field splittings. Clearly magnetic order is consistant with a heterogeneous mixture of ~ m ' + and Tm3+. Varma [7] has suggested that in the case of Tm, where both ions may be magnetic the homogeneous intermediate valence state can order magnetically. The most detailed information on the magnetic structure comes from the neutron diffraction data of Shapiro et al. [S]. For a large lattice constant sample, 5.71 A, the order is type I antiferromagnetic with an ordered moment of 1.7 p,. This value is consistant with 2 ,uB from high field measurements 191. The small moment compared with the free ion values of 4 and 7 p, of Tm2+ and Tm3+, respectively, indicates that there is some interaction which splits the Hund's rule states. Shapiro et al. [8] interpret the low moment in terms of a homogeneous intermediate valence. Although this argument is perfectly plausible it does not exclude crystal field effects in addition to or instead of intermediate valence effects. They have also measured a sample with a small lattice constant (5.63 A), and find type 11 antiferromagnetic ordering but only extending over regions of about 100 A. The magnetic moment reported is 0.5 p,. This result is understandable, since in this sample Tm is essentially trivalent, the moment is exchange induced and the large vacancy concentration interferes with long range order. The neutron diffraction measurements on the 5.71 A sample, as a function of temperature and magnetic field, have established that the magnetic phase diagram developed by Ott, et al. [l01 and Guertin, et al. [l l ] consists simply of an antiferromagnetic and a metamagnetic phase transition. Mgller et al. [l21 have successfully modeled the magnetic system on the basis of mean field theory including cubic anisotropy and first and second neighbor Heisenberg exchange. The success of the mean field approach is consistant with the existence of a homogeneous intermediate valence but does not exclude inhomogeneities in the real system. It is evident from the discussion above that the physical properties are strongly sample dependent. As a consequence we have undertaken a study of the solid state chemistry of TmSe in relation to its physical properties [13]. A similar investigation has been made by Batlogg et al. [14]. A series of single crystals of Tm,Se were grown with varying concentration, X, including some samples prepared from large excesses of Tm metal. The resulting crystals 'have a color range from bronze, at the nominal stoichiometric concentration, to a metallic blue for large Tm vacancy concentrations, similar to the Gd,Se system 1151. The samples were generally slowly cooled from 2 200-2 400 oC, depending on starting concentration, to 1 600-1 800 OC for annealing and then rapidly cooled to room temperature. Unlike most other crystals of the monochalcogenides, the TmSe crystals do not cleave cleanly along (100) planes and generally show some surface curvature. It was, however, possible to find regions of an ingot with relatively parallel cleavage. The samples were analyzed wet chemically using ethylene diamine tartrate for the rare earth determination. The selenium was analyzed with a permanganate oxidation and back titration. Figure 1 is a plot of lattice constant, a,, as a function of concentration, X, in Tm,Se. Near the stoichiometric composition the lattice constant is 5.71 A which corresponds to a valence of 2.77, assuming a linear interpolation between 5.64 A for Tm3+Se and 5.95 A for Tm2+Se. The rapid decrease of lattice constant with decreasing Tm concentration demonstrates that the increase in Tm vacancies reduces the amount of Tm2 +. In order to separate the effect of vacancies and changing valence we can compare the results with those obtained on Gd,Se [l51 for which there is no valence change. The overall decrease in lattice constant in this system is0.02 A for approximately the same limiting concentration. A linear interpolation between Er and Lu selenides fixes the lattice constant of C5-316 F. HOLTZBERG, T. PENNEY AND R. TOURNIER 5.62 0.7 0.8 0.9 1 .O 1.1