Suppression of Ru (S = 1) spin dimerization in La₂RuO₅ by Ti substitution

Abstract

La2RuO5 shows a magneto-structural phase transition at 161 K with spin dimerization and concomitant formation of a non-magnetic singlet ground state. To gain a deeper insight into the origin of this transition systematic substitution of Ru by Ti has been carried out. Polycrystalline samples have been synthesized by thermal decomposition of citrate precursors leading to La2Ru(1-y)Ti(y)O5 (0 ≤ y ≤ 0.45). The crystal structure was investigated by x-ray powder diffraction at room temperature and at 100 K. The valences of Ti and Ru were obtained from x-ray absorption near edge structure spectroscopy at the Ti-K and the Ru-LIII absorption edges, respectively. The magnetic phase transition was investigated by magnetic susceptibility measurements as a function of Ti substitution, revealing a decreasing transition temperature on increasing the level of substitution. The step-like feature in the magnetic susceptibility reflecting the Ru-Ru spin dimerization transition becomes smeared out close to y = 0.3 and completely vanishes at y = 0.45, indicating complete suppression of spin-dimer formation. Additional specific-heat measurements show a continuous decrease of the magnetic entropy peak with increasing Ti substitution mirroring the reduced number of spin dimers due to the magnetic dilution. A magnetic anomaly of the dimerization transition can hardly be detected for y ≥ 0.3. Density functional theory calculations were carried out to study changes of the electronic band structure caused by the substitution. A possibly preferred distribution of Ti and Ru and the magnetic interactions as well as the change of the density of states close to the Fermi level are investigated. Based on these experimental results a detailed (y,T) phase diagram is proposed.

Synthesis, crystal structure, and valence states of Mn-substitutedLa2RuO5

“…12). This is a typical behavior caused by the increasing rigidity of solids at low temperatures as already observed for La 2 Ru 0.55 Ti 0.45 O 5 [28]. From the linear part above 80 K the thermal expansion was determined to α c = 20 × 10 −6 K −1 , which is in the typical range for oxides.…”

“…[22,28] [20,22,28] and is caused by smaller La sites than calculated from typical interatomic distances of La 3+ and O 2− . For the LaRuO 4 layers the absolute values of the valences were slightly smaller than expected: The valences Ru +3.9, La +2.9, and O −1.9 reflect the opposite effect of smaller anion and larger cation sites.…”

“…The substitution of Ru by Ti (La 2 Ru 1−y Ti y O 5 ) leads to a progressively decreasing Ru-spin dimerization temperature, which becomes fully suppressed for y = 0.45, the highest achievable substitution level [28]. The Ti incorporation directly interrupts the antiferromagnetic Ru-Ru spin coupling.…”

“…[22] and [28]. A precursor solution was prepared by dissolving stoichiometric amounts of La(NO 3 ) 3 × 6 H 2 O (Chempur, 99.9%), ruthenium nitrosyleacetate (Alfa Aesar, 99.99%), Mn(NO 3 ) 2 × 4H 2 O (Fluka, p.a.…”

“…From the observed changes it can be expected that also the low-temperature crystal structure as well as the phase transition are significantly affected. The structural transition temperature, which is directly linked to the dimerization [22,28] To study the effect of Mn substitution on the structural phase transition, the cell parameters at room temperature and 100 K were compared. In pure La 2 RuO 5 the values for a, b, and α decrease during the transition from the ht to the lt modification, while c, β, and γ increase.…”