Journal of the Physical Society of Japan
Vol. 70, No. 8, August, 2001, pp. 2327{2332
X-Ray Structural Study of Ferroelectric KH2AsO4 and KD2AsO4
1
1
*
Mizuhiko ICHIKAWA , Daisuke AMASAKI**, Torbjorn GUSTAFSSON and Ivar OLOVSSON
¨
Division of Physics, Graduate School of Science, Hokkaido University, Sapporo 060-0810
ꢀ
Materials Chemistry, Angstrom Laboratory, Uppsala University, Box 538, SE-751 21 Uppsala, Sweden
1
¨
(Received April 11, 2001)
In order to understand systematically the relationship between the crystal structure and phase
transition in tetragonal KH2PO4 (KDP) family, the structures of KH2AsO4 (KDA) and KD2AsO4
(DKDA), which are the end members concerning the transition temperature Tc, have been studied
by X-ray diꢀraction at room temperature and at Tc þ 5 K (110 K and 171 K, respectively). It was
revealed that the hydrogen-bond distances ROO for KDA and DKDA are longer than those for
KDP and DKDP, respectively. These results smear the linear ROO vs Tc relation found earlier in
the tetragonal KDP family, contrary to our expectation. The temperature dependence of the bond
lengths and angles in KDA and DKDA are compared with those in other members in the family
and similarity and diꢀerence are discussed.
KEYWORDS: X-ray diꢀraction, KDP-type crystal, hydrogen-bonded ferroelectrics, transition temperature, isotope eꢀect
temperature and at Tc þ 5 K (110 K and 171 K respec-
x1. Introduction
tively) in order to study systematically the relationship
Tetragonal KH2PO4 (KDP) type crystals (Fig. 1) between crystal structure and phase transition in the
constitute a group of typical hydrogen-bonded ferro- tetragonal KDP family. The purpose of this paper is to
electrics. The member crystals belonging to this report these results and to discuss the temperature
MH2XO4 family (M ¼ K, Rb, Cs, NH4; X ¼ P, As) have dependence of the bond lengths and angles in comparison
the following characteristics: (1) they are isomorphous, with those in other members of the family.
ꢁ
with the space group I42d in the room temperature phase
and (2) the protonated compounds exhibit a large
isotopic upward shift (ꢀ 100 K) of the transition
x2. Experimental
KH2AsO4 was synthesized by the reaction K2CO3þ
temperature Tc when deuterium is substituted for As2O4 þ 2H2O ! 2KH2AsO4 þ CO2. Single crystals
hydrogen.1) The origin of this remarkable isotope eꢀect were grown by cooling the saturated solution around
and phase transition mechanism have attracted much 313 K. KD2AsO4 crystals were similarly obtained from
interest and extensive experimental and theoretical cooling the saturated D2O solution. As-grown crystals
studies have been devoted to elucidate them.2{4) This with well developed natural faces were selected as
group of crystals, which includes many isomorphous specimen. All faces of the specimen were indexed and
members, is exceptional in ferroelectric compounds since the distances to each face from the origin placed at an
a crystal often changes its structure by changing part of intersecting point of three faces were measured. The
the constituent atoms. Tetragonal KDP-type crystals maximum origin to face distance of the specimen was
therefore oꢀer a unique opportunity to investigate 0.15 mm for KH2AsO4 and of 0.25 mm for KD2AsO4.
systematically the relationship between phase transition Intensity data were collected using Mo Kꢀ radiation
ꢀ
and structure. The crystal structure of the KDP family, (ꢁ ¼ 0:71073 A) on a Huber/Stoe/Aracor four-circle
in particular KDP and DKDP, has been extensively diꢀractometer equipped with a 400 mm ꢂ-circle and
investigated by X-ray diꢀraction as well as neutron two-stage closed-cycle helium refrigerator.9) The data
diꢀraction.5,6) It is known that the transition tempera- collection was done over one hemisphere of the reciprocal
ture is aꢀected by the change of atoms at the M and X space. The other experimental details are given in
sites, although the deuteration eꢀect is the most Table I.
remarkable (e.g. ref. 7).
It was necessary to know Tc of the specimen used to
KH2AsO4 (KDA) and KD2AsO4 (DKDA) also belong collect the data just above Tc, since Tc depends on the
to the tetragonal KDP family and have the lowest crystal quality as well as the deuterium content in the
transition temperature among the members (97 K and case of deuterated crystals. Tc of the sample was
162 K, respectively).1) The structure of KDA, however, estimated from an average of the highest transition
has been studied much less than that of KDP8) and the temperature on cooling and the lowest transition
structure of DKDA has not yet been reported. Due to temperature on heating during several thermal cyclings,
these circumstances the structures of KDA and DKDA since tetragonal KDP family is known to exhibit a ꢁrst-
have been determined by X-ray diꢀraction at room order transition. The transition temperature was judged
from the peak-splitting of the intensity proꢁles due to
the formation of ferroelectric domains below Tc. The
estimated Tc was 105(1) K for KH2AsO4 and 166ð1Þ K
for KD2AsO4. Thus the temperature at Tc þ 5 K for
*Corresponding author: Tel.: +81-11-706-4416; Fax: +81-11-706-4926.
E-mail: ichikawa@phys.sci.hokudai.ac.jp
**Present address: Dentsu Hokkaido, Sapporo 060-8545.
2327
Ichikawa, Mizuhiko
