Surfaces, Interfaces and Films

Characterisation and phase transitions in a new mixed acid sulphate K_0.9Rb_0.1HSO₄

H. Guermazi^a1, M. Bouachir^a2, Y. Mlik^a1 and T. Mhiri^a3

Département de Physique, Institut préparatoire aux études d'ingénieurs de Sfax, Route Menzel Chaker km 0.5, BP 805, 3018 Sfax, Tunisia

Département de Physique, Institut préparatoire aux études d'ingénieurs de Nabeul, Tunisia

Laboratoire de l'état solide, Faculté des Sciences de Sfax, 3038 Sfax, Tunisia

Abstract

A new mixed acid sulphate K_0.9Rb_0.1HSO₄ was synthesized at room temperature. It has been firstly characterised by X-ray diffraction (XRD) and vibrational spectroscopy investigations. Then, in order to detect phase transitions and watch changes in the conductivity behaviour, investigations by differential scanning calorimetry (DSC), (XRD), electrical conductivity measurements, and impedance spectroscopy were carried out. The XRD results on single crystals showed that at room temperature, K_0.9Rb_0.1HSO₄ has an orthorhombic symmetry with space group Pbca. The unit cell parameters are: a = 8.39(6) Å; b = 9.79(8) Å and c = 18.98(5) Å. The Raman and infrared spectra at room temperature were also investigated. The structural similarity with KHSO has been confirmed. The thermal studies showed that K_0.9Rb_0.1HSO₄ undergoes two structural phase transitions at about 400 K and 435 K. A partial substitution of K⁺ by Rb⁺ increases the conductivity even at room temperature. The transition at 435 K leads to a super-ionic conductor state at high temperatures related with rotational motions of HSO₄ ⁻ ions.

(Received September 25 2001)

(Revised January 31 2002)

(Accepted February 21 2002)

(Online publication May 15 2002)

PACS:

• 78.30.-j – Infrared and Raman spectra;
• 77.80.Bh – Phase transitions and Curie point;
• 68.35.Rh – Phase transitions and critical phenomena

hj.tounsi@ipeis.rnu.tn