Nanocomposites and nanomaterials
Peculiarities of thermal expansion of α-(BEDT-TTF)2NH4Hg(SCN)4 single crystal

R.M. Basnukaeva1, A.V. Dolbin1, M.V. Khlistyuck1, V.B. Esel'son1, 
V.G. Gavrilko1, N.A.Vinnikov1

1 B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Nauky Ave., Kharkov 61103, Ukraine.
E-mail:razeta9556@gmail.com
The problem of synthesis and study of the physical properties of low-dimensional organic molecular conductors and magnetic materials is at the junction area of solid state physics, materials science and organic synthesis. Linear thermal expansion of α-(BEDT-TTF)2NH4Hg(SCN)4 single crystal in the direction of the crystallographic c axis (along the crystal layers) has been investigated in the temperature interval 2–250 K. The study was carried out using a capacitive low-temperature highly sensitive dilatometer [1]. The temperature dependence of the LTEC of α-(BEDT-TTF)2NH4Hg(SCN)4  havn’t pronounced peaks observed in the LTEC curves of the κ-(D4-BEDT-TTF)2Cu[N(CN)2]Br single crystals and κ-(BEDT-TTF)2Cu[N(CN)2]Cl  [1,2]. However, there were a number of features. First of all, Below 3.5-4 K LTEC α-(BEDT-TTF)2NH4Hg(SCN)4 has small negative values. Perhaps the reason is superconductive fluctuations coupled with charge disproportionation. There were a number of anomalies (bends) of the dependence α(T) in the temperature range T *(25-45 K), the magnitude of these anomalies exceeds the experimental error. Perhaps these anomalies are explained by order-disorder arrangements of NH4+. The hump structure at 200 K is probably related to this since the charge diproporationaion itself is electronic origin but should have large change in phonon structure. Also the dependence of α (T) was observed in the temperature range 200-220 K, what is likely characterized by the changing in the slope of the α (T) dependence and an increasing scatter of the experimental values. Perhaps it is related to the freezing of the rotational motion of NH4+ ions coordinated by four SCN ligands [3].

1. Dolbin A.V., Khlistyuck M.V., Esel’son V.B., Gavrilko V.G., Vinnikov N.A., Basnukaeva R.M., V.V. Danchuk, Konstantinov V.A., Nakazawa Y. Peculiarities of thermal expansion of quasi-two-dimensional organic conductor κ-(BEDT–TTF)2Cu[N(CN)2]Cl // Low Temp. Phys. – 2016. – 42. – P.788–793.
2. Dolbin A.V., Khlistyuck M.V., Esel’son V.B., Gavrilko V.G., Vinnikov N.A., Basnukaeva R.M., Konstantinov V.A., Nakazawa Y. // Low Temp. Phys. – 2017. – 43, P. 1740–1744.
3. Endo S., Watanabe Y., Sasaki T., Fukase T., Toyota N. Temperature dependence of lattice parameters of α-(BEDT-TTF)2MHg(XCN)4 (M=K, Rb,NH4, and X=S, Se), Synth.Met. – 1997. – 86. – P. 2013-2014.