Nanoscale physics

Mechanical properties of Cu6PS5I-based argyrodite thin films
Bilanych V.V., Bendak A.V., Skubenych K.V., Bilanych V.S., Studenyak I.P.
Uzhhorod National University, Pidhirna Str., 46, 88000, Uzhhorod, Ukraine. 
E-mail: vvbilanych@gmail.com

Cu6РS5I compound belongs to the superionic conductors with argyrodite structure. Due to the high electrical conductivity they are the promising materials for the wide application as the solid electrolytes, supercapacitors, ion-selective membranes, and others electrochemical devices. The aim of the present paper is to investigate the hardness and Young's modulus dependences on penetration depth in Cu6PS5І-based thin films as well as hardness dependences on chemical composition of Cu6PS5І-based thin films.
Cu6PS5І-based thin films were deposited onto silicate glass substrates by non-reactive radiofrequency magnetron sputtering. The chemical composition of the thin films with different copper content was determined using energy dispersive X-ray spectroscopy. The hardness measurements of Cu6PS5І-based thin films were performed using PMT-3 microhardness meter at room temperature.
The hardness H and Young’s modulus E dependences on the penetration depth h were studied for Cu6.43P1.15S4.64I0.83 thin film. The rapid increase of the H and E parameters in the range h=20-150 nm was observed while at nm these parameters increase less intensively. To explain the H(h) and E(h) dependences we can use a model of soft thin film on a rigid substrate. The rapid increase of the H and E parameters in the interval h=20-150 nm, obviously, is associated with the gradual formation of deformation zones under the indenter. In the range of 150-250 nm the advancement of local deformation zones into the depth of the film is observed. The increase at the H(h) dependence at nm is revealed which indicates that the deformation zones achievement the substrate. 
The compositional studies of Cu6PS5І-based thin film have shown that he increase of the copper content leads to decreasing of the films hardness almost in 2 times. At the same time, the conductivity of thin films increases. It is well known that high ionic conductivity of the investigated thin films is due to the formation of conduction channels such as dendrites and clusters at the high copper content. This phenomenon may be related to the nano-separation in the amorphous film. The structure of the thin film can be distinguished by rigid clusters and relatively mild inter-cluster volume. The displacement of rigid clusters comparative to the soft environment due to indenter immersion is occur. The hardness of the film is determined predominantly by the stiffness of the inter-cluster volume.