AC Conductivity and Dielectric Properties of Lepidocrocite-type Alkali Titanate Tunable by Interlayer Cation and Intralayer Metal

Abstract

The lepidocrocite-type layered alkali titanate A x M y Ti 2−y O 4 has diverse chemical compositions with variation in charge per formula unit x, the interlayer cation A + , and the intralayer metal M. Despite this multivariable nature, the composition dependence of physical properties is not well explored. We report herein the AC conductivity and the complementary dielectric properties of Cs 0.7 M 0.35 Ti 1.65 O 4 , K 0.8 M 0.4 Ti 1.6 O 4 (M = Zn, Ni), and the mixed-interlayer ion Cs 0.6 K 0.1 Zn 0.35 Ti 1.65 O 4 . For Cs 0.7 Zn 0.35 Ti 1.65 -O 4 , the total AC conductivity is ∼7 × 10 −8 to 2 × 10 −6 S•cm −1 at 200−350 °C, associating with an activation energy E a ∼ 865 meV. Meanwhile, the conductivity of K 0.8 Zn 0.4 Ti 1.6 O 4 is higher by 1 order of magnitude at much lower temperature (25−150 °C) and a smaller E a ∼ 250 meV. This difference originates from the compositional robustness of the cesium-containing samples, contrasting with the sintering-induced changes in the potassium analogues. For the latter, the loss of the interlayer K + ion results in (i) generation of carriers due to charge compensation, (ii) reduction of sheet charge density and weakening of electrostatic attraction, and (iii) widening of the interlayer distance, all contributing to a lower E a in K 0.8 M 0.4 Ti 1.6 O 4 . The angular frequency dependence of conductivity, dielectric permittivity (up to a colossal value of 10 9 ), and dielectric loss follows the universal power law. Our work demonstrates the potential of simple compositional variation for electrical properties tuning, prompting a more in-depth investigation covering a wider range of possible candidates of x, A + , and M in lepidocrocite titanate.

Effect of Adsorbed Water and Temperature on the Universal Power Law Behavior of Lepidocrocite-Type Alkali Titanate Ceramics

“…The stoichiometric mixture of Cs 2 CO 3 (and/or KHCO 3 ), ZnO, TiO 2 , and a small amount of 1/1 w/w ethanol/water was vibratory-milled for 2 h prior to washing and drying at 80 °C for 24 h . The mixture was calcined at 900 °C for 20 h to produce the respective polycrystalline sample of the lepidocrocite alkali titanate.…”

“…Next, an amount of ∼0.6 g of the powder (previously mixed with a small amount of polyvinyl alcohol binder) was pressed into a ∼1.1 cm wide and ∼0.2 cm thick circular pellet. This green compact was then heated at 500 °C for 2 h to drive off the organic binder prior to sintering at 1100 °C for 4 h for K 0.8 Zn 0.4 Ti 1.6 O 4 (84% density) and Cs 0.7 Zn 0.35 Ti 1.65 O 4 (89%) . The mixed interlayer ion Cs 0.6 K 0.1 Zn 0.35 Ti 1.65 O 4 requires two cycles of sintering at 1100 °C for 4 h and an additional annealing at 800 °C for 20 h to give a specimen with 72% density .…”

“…This green compact was then heated at 500 °C for 2 h to drive off the organic binder prior to sintering at 1100 °C for 4 h for K 0.8 Zn 0.4 Ti 1.6 O 4 (84% density) and Cs 0.7 Zn 0.35 Ti 1.65 O 4 (89%) . The mixed interlayer ion Cs 0.6 K 0.1 Zn 0.35 Ti 1.65 O 4 requires two cycles of sintering at 1100 °C for 4 h and an additional annealing at 800 °C for 20 h to give a specimen with 72% density . Then, the top and bottom surfaces of the pellets were polished, applied with a silver paste (Heraeus c1000), and fired at 750 °C for 20 min.…”

“…It is well known that alkali-containing ceramics are prone to atmospheric water adsorption. Among these, several members of the alkali titanate family have received increasing attention thanks to their superionic conduction, giant/colossal dielectric permittivity, ,, and water-induced transport properties. − The alkali titanates show rich structural chemistry and compositional flexibility, as exemplified by the layered or tunnel-structured Andersson–Wadsley-type A 2 O· n TiO 2 (A = alkali; 1 ≤ n ≤ 6) and the lepidocrocite-type layered A x Ti 2– y M y O 4 (M = transition metals of the first row). − In particular, the layered alkali titanates can undergo exfoliation into nanosheets, which are an important component in nanoelectronics or triboelectric nanogenerators. − In these cases, water can be incorporated either as (i) crystalline hydrates at the interlayer space, , (ii) noncrystalline hydrates at the grain boundary/porosity/external surfaces, ,, or (iii) the combination of these two. , Investigating the effects of noncrystalline water will widen our fundamental understanding of water-induced conductions, complementing extensive works on the crystalline hydrated (i.e., protonic) titanates ,,, and the nanosheet analogues, , offering ways to enhance the sensing performances, and so on.…”

“…Very recently, we have reported the structural/compositional aspects and electrical properties of some lepidocrocite-type alkali titanates at 25 and 300 °C. In this article, we reported in more detail the follow-up experiments focusing on the effect of the unintentionally adsorbed atmospheric water on the electrical properties of K 0.8 Zn 0.4 Ti 1.6 O 4 , Cs 0.7 Zn 0.35 Ti 1.65 O 4 , and Cs 0.6 K 0.1 Zn 0.35 Ti 1.65 O 4 .…”