New scandium rhodium boride Sc4Rh17B12 with a framework structure: synthesis, crystal structure, and properties

Abstract

The new scandium rhodium boride Sc 4 Rh 17 B 12 was synthesized by arc melting of the elements followed by annealing in inert atmosphere. The crystal structure of Sc 4 Rh 17 B 12 was solved using single crystal X ray diffraction data. The structure can be described as a three dimensional frame work formed by trigonal prisms [BRh 6 ] and [BRh 5 Sc] with isolated boron atoms inside the prisms and trigonal prisms [BRh 5 B] representing the coordination polyhedra of paired boron atoms. The temperature dependences of the magnetic susceptibility and specific resistance of Sc 4 Rh 17 B 12 revealed that the compound is a Pauli paramagnet and shows metal like specific resistance.Key words: Sc 4 Rh 17 B 12 , synthesis, crystal structure.Owing to the specific electronic structure, scandium is often assigned to rare earth elements. However, due to the small atomic radius of Sc (r Sc = 1.606 Å) 1 close to the ra dii of alkaline earth metals (for example, r Mg = 1.600 Å), 1 the crystal chemistry of scandium compounds is unique in many respects. Thus ternary scandium and platinum metal borides tend to exist as both structural analogs of rare earth element borides or compounds isostructural to complex magnesium borides and as unique structural types, which are often related to transition metal com pounds.At low boron content (M/B > 4, where M is the total quantity of metals in the compound, M = Sc + T) in Sc-T-B systems, where T = Ru, Rh, Re, Os, Ir, perovs kite like compounds ScT 3 B 1-x , 2-4 typical of lanthanide containing systems, are formed. These compounds can be considered as interstitial solid solutions containing boron in the octahedral cavities of ScT 3 intermetallics. As the boron content increases to the ratio 1 ≤ M/B ≤ 4, phases with structures identical to the structures of the corresponding magnesium borides are formed (ScOs . 7)). Also, among the ternary scandi um and platinum metal borides in the composition re gion of 1 ≤ M/B ≤ 2, there are compounds that form their own types of structure. These structures are often derived from ternary rare earth element borides and are formed as a result of change in the composition or distor tions associated with the small atomic radius of scan dium (ScIr 3 B 4 , 9 ScOsB 2 , 10 ScIr 3 B 2 , 11 Sc 2 (Os,Ru) 5 B 4 (see Ref. 12)). In the boron rich region, there exist phas es Sc 2 TB 6 5,13-14 crystallizing in the Y 2 ReB 6 structural type, 15 which is frequently encountered among rare earth element borides.Considerable researchers´ interest has always been at tracted by compounds of the ET 4 B 4 class (E is a rare earth metal) in the Sc-T-B systems with 1 ≤ M/B < 2.

Metallic Borides, La2Re3B7 and La3Re2B5, Featuring Extensive Boron–Boron Bonding

“…La 2 Re 3 B 7 is metallic, as exhibited by the increase in resistivity with increasing temperature. The room-temperature resistivity, ρ 300 K , is ∼375 μΩ cm, which is comparable to the values observed for some other metal borides, including 121 μΩ cm for Mg 3 Rh 5 B 3 , ∼250 μΩ cm for Ru 7 B 3 , ∼320 μΩ cm for Mg 1– x RhB, and ∼420 μΩ cm for Sc 4 Rh 17 B 12 . The somewhat large residual resistivity (ρ 0 ≈ 100 μΩ cm) may be due to pitting in the crystal due to soaking it in dilute HCl to clean the excess flux from the surface.…”

“…The room-temperature resistivity, ρ 300 K , is ∼375 μΩ cm, which is comparable to the values observed for some other metal borides, including 121 μΩ cm for Mg 3 Rh 5 B 3 , 69 ∼250 μΩ cm for Ru 7 B 3 , 70 ∼320 μΩ cm for Mg 1−x RhB, 71 and ∼420 μΩ cm for Sc 4 Rh 17 B 12 . 72 The somewhat large residual resistivity (ρ 0 ≈ 100 μΩ cm) may be due to pitting in the crystal due to soaking it in dilute HCl to clean the excess flux from the surface. However, the large residual resistivity and small residual resistivity ratio (RRR = ρ 300 K /ρ 0 ≈ 3.75) may also indicate a high degree of intrinsic structural disorder (point defects) in the complex unit cell.…”