Complex physical properties of EuMgSi – a complementary study by neutron powder diffraction and 151Eu Mössbauer spectroscopy

Abstract

X-ray pure samples of EuMgSi were synthesized by reactions of the elements in sealed niobium tubes using a high frequency and subsequently a resistance furnace. The structure was investigated by single crystal X-ray diffraction: TiNiSi-type, Pnma, a = 769.5(2), b = 455.0(1), c = 836.9(2) pm, wR₂ = 0.033 [I ≥ 2σ(I)], and 705 F² values with 20 variables. Powder synchrotron radiation diffraction experiments did not reveal any structural changes down to 4.3 K. Magnetic susceptibility data and ¹⁵¹Eu Mössbauer spectra clearly indicate a stable Eu²⁺ configuration. Two distinct magnetic anomalies around 12 and 14 K can be observed for different samples with dc- and ac-susceptibility, heat capacity and resistivity measurements. Fitting of hyperfine field splitting as a function of temperature (¹⁵¹Eu Mössbauer spectroscopy data) with a Brillouin function also leads to a magnetic ordering around 14 K. Electronic structure calculations in coincidence with the resistivity measurement prove narrow (or nearly zero) gap-semiconducting behaviour. The calculated band gap energy of 0.03 eV should be considered with precautions due to the accuracy of this method. An incommensurate magnetic structure with the propagation vector k = [q_x ≈ 0.37, 0, 0] was determined using neutron diffraction data at 5.5 K. In consensus of dc- and ac-susceptibility and neutron powder diffraction a complex combination of antiferromagnetic and ferromagnetic interactions, most likely by super-exchange, is confirmed. These cause two magnetic ordering temperatures, though only one independent crystallographic Eu site in terms of the crystal structure is present in EuMgSi.