Structural Study of TlH2PO4 and TlD2PO4 in the High Temperature Phase

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Structural Study of TlH2PO4 and TlD2PO4 in the High Temperature Phase

S. Rios, W. Paulus, A. Cousson, M. Quilichini, G. Heger, N. Le Calvé, B.

Pasquier

To cite this version:

S. Rios, W. Paulus, A. Cousson, M. Quilichini, G. Heger, et al.. Structural Study of TlH2PO4 and

TlD2PO4 in the High Temperature Phase. Journal de Physique I, EDP Sciences, 1995, 5 (7), pp.763-

769. �10.1051/jp1:1995166�. �jpa-00247100�

Classification Physics Abstracts

61.12 77.80

Short Communication

Structural Study of TIH2P04 and TID2P04

in the High Temperature Phase

S. Rios

(1),

W. Paulus _(1>~), ^{A. Cousson} _(~

),

M.

Quilichini

_(~

),

G.

Heger (3),

N. Le Calvé (~) and B.

Pasquier

_(~)

(~) Laboratoire Léon Brillouin, CE Saclay, 91191 Gif-sur-Yvette Cedex, France

(~) Philipps-Universitàt Marburg, Fachbereich Chemie, Hans-Meerwein Strafle, 3~ 632 Marburg, Germany

(~) ^RWTH Aachen, Institut für

Kristallographie,

Jàgerstr. 17-19, 52052 Aachen, Germany

(~) Laboratoire de Spectrochimie Infrarouge et Raman, CNRS, 2 _rue Henri Dunant, 94320

Thiais, France

(Received 20 February1995, revised 17 May1995, accepted 22

May1995)

Résumé. Les _structures cristallines de TIH2P04 et TID2P04 ont été déterminées à 373 K par diffraction de neutrons sur des échantillons monocristallins. L'acquisition de données est faite _{avec un} diffractomètre 4-cercles, _en utilisant la longueur d'onde

= 0.830 À. A cette

température les deux composés ont la même symétrie orthorhombique décrite dans le groupe

d'espace Pbcn. Cette phase ^haute température apparaît _comme la phase "mère" du diagramme de phase obtenu _pour des températures décroissantes _pour chacun des deux

comi,, sés TDP et

DTDP.

Abstract. The crystal structures of bath TIH2P04 and ils deuterated form '. 12P04 bave been studied at 373 K _using single-crystal _neutron diffraction. Data _were colt, d _{on a} four- circle diflractometer at

= 0.830 À. _{At this} temperature bath compounds bave trie _{same or-} thorhombic structure, space group Pbcn, which is trie parent phase ofthe two diflerent _sequences of phase transitions observed in TDP and DTDP

on decreasing temperature.

1. Introduction

TIH2P04

(TDP)

and ils deuterated form

TID2P04 (DTDP) belong

to trie

family

of ferroelec- tric materials of

KH2P04 (KDP).

This

family

of

compounds,

_on

deuteration,

show _a

large isotopic

^eflect in their ferroelectric transition temperatures, Tc. Since the

discovery

of ferro-

electricity

in

KDP,

dilferent kinds of studies have been carried out for

KDP, CSDP, RbDP,

ADP and their deuterated forms with the purpose of

understanding

this

isotopic

^eflect.

© Les Editions de Physique 1995

764 JOURNAL DE PHYSIQUE I N°7

From calorimetric and dielectric measurements

il,2],

TDP is known to

undergo

_a ferroelastic

phase

transition at

T

= 357

K,

and _an antiferroelectric _{one at}

T)

= 230 K. While tue

crystal

symmetry at _room temperature is

monoclinic,

_{space group}

P21la [3,4],

recent

X-ray

_mea-

surements bave suown tuat above T ^it is

ortuoruombic,

Pcan [5], ^{wuicu is} a non-conventional

setting

of Pbcn. For tue deuterated

compound,

dielectric and

polarization

measurements

[6,7]

show

only

_an antiferroelectric

phase

transition around

Tf

= 353 K

(as

suown in [8], ^T~

depends

on tue

degree

^of

deuteration). Tuus,

for tue

TDP/DTDP

system ^{tue suift} in T~ is

Ai

₌ 123 K.

Concerning crystal

structure,

uowever,

there is little literature about tue deuterated

form,

and tue

reported

_one

[9-11]

is ratuer controversial. In order to understand the

phase

transitions in the

TDP/DTDP

system, we have undertaken _a systematic structural

study

of botu _com-

pounds

at diflerent temperatures. ^We present uere in

particular

_{some new} results of the

high

temperature

phase (T

> T for TDP and T >

Tf

for

DTDP),

_as well _as their

surprisingly

diflerent beuaviour

concerning

tueir

phase

transitions with

decreasing

temperature.

2.

Experimental

2.1. DATA COLLECTION _AND REFINEMENT. TO determine trie structure of trie

high-

temperature

phase

for botu TDP and

DTDP,

we used

single-crystal

neutron diffraction. The dimensions of tue

single-crystals

_were about 10 mm3. Data _were collected for botu

compounds

at 373 K _on tue four-circle dilfractometer Pi10

(beamline 5C.2)

_at the Reactor

Orphee, Saclay (France),

_at

= 0.830

À.

_The temperature

stability

in the home-made

sphere

fumace _was better than 373 + 0.25 K. Details about the data collection and refinement _are

given

in Ta- ble I. In the _case of

TDP,

intensities _were corrected from

absorption

elfects

(/J

₌ 0.82

cm~~,

experimentally determined).

Structure refinements _were carried out

using

tue _program CRYS- TALS [12].

Starting

parameters ^for least _squares refinements _were obtained

by

direct metu- ods

(SHELXS [13])

in tue _case of

DTDP,

wuereas for TDP tue _room temperature struc-

ture data _were used. Tue

weiguting

scheme used in tue last refinement

cycle

_{was W}

=

W'

(1- (AF/6a(Fobs))~)~

where W'

=

1/L(A,Ti(x))

witu turee

coefficients, A;,

^for the

Cuebyshev polynomial A,T,(x),

_x

being Fc~ic/F~~ic (max).

Positional and

anisotropic

thermal pararneters, overall scale factor and

isotropic

extinction parameter were refined _a total of 41 parameters.

In the _case of DTDP tue deuterium site occupancy was also included _{as a} variable parameter.

Tue value obtained

corresponds

to _a

degree

of deuteration > 98%.

2.2. CRYSTAL STRUCTURE _OF

TÎH2PÙ4

_AND

TID2PÙ4

IN THEIR HIGH TEMPERATURE

PHASE. At 373

K,

TDP and DTDP _were found _to bave tue _same ortuoruombic struc-

ture, space group Pbcn. Tuallium and

puospuorus

atoms _are situated _on tue two-fold axis

parallel

to tue ~axis, wuereas _oxygen atoms occupy

general positions.

P04 units _are linked to-

getuer by

two

crystallograpuically

^dilferent types ^of

H(D)

atoms. Tue first type,

Hi (Di)>

links

P04

_groups

forming

cuains

along

tue c-axis. Tuese cuains _are linked

parallel

to tue a-axis

by

tue second type ^of

H(D)

atoms,

H2(D2)> creating

sueets

perpendicular

to tue ~axis.

Tuus,

in

TDP and DTDP tue

uydrogen

^{bonds form} a two-dimensional network. In _a first step, Hi

(Di)

atoms were

placed

_{on a} centre of inversion and

H2(D2)

atoms on tue two fold axis

parallel

to tue

~axis,

_1-e-, in botu _cases

exactly

in tue middle of tue two oxygen atoms of

adjacent P04

tetraedra. Tuis model revealed

uiguly anisotropic

thermal parameters in the

O-H(D)-O

direction for both types of

H(D)

atoms.

Large anisotropic

thermal parameters _were ^{also found} for ail oxygen ^atoms. Table II and III show

positional

and anisotropic thermal parameters for TDP and DTDP. Dur results for TDP _were found to be _in agreement ^with those obtained in _a

previous X-ray study

_[5]. Distances between

adjacent

_oxygen atoms,

O-O, O-H(D)

and

P-O distances _{are given} in Table IV.

Figure

1 shows trie _structure of trie isostructural

high

Table I. Data collection and

refinement

values

for

TDF and DTDP.

TDp DTDp

Space Group Pbcn

Temperature 373 K

sm0~~/À~À-1)

0.710 0.729

Cell Parameters (À) _a

= 4.534(7) _{a =} 4.535(4)

b = 14.38(2) b

= 14.36(1)

c = 6.54(1) _c

= 6.556(5)

Z 4 4

Number of

measured reflections 689 1620*

Number of 455 (R,~, = 2.19b) 409 (R ,~,.69b)

unique reflections (228 1> 20E(Il) (291 1> 36(Il)

R (Rw 0.044 (0.049) o-Mo (0.047)

S (goodness offit) 1.2 1.2

*Upto four equivaJents (instead of two as in trie _case ofTOP) were measured for each reflection.

temperature

phase

of TDP and DTDP.

In order _to

analyse

those

large anisotropic

^thermal parameters, we introduced _a

split position

model for ail

H(D)

atoms, which

yielded

_a structure model with reasonable temperature factors and

O-H(D)

distances in trie order of1.0

À.

_{Trie distance between trie two}

split positions

_was

r- 0A

À.

_However,

no

significant improvement

in the refinement _was achieved. Due to the

crystal

size and trie reduced

scattering

_power at

high scattering angles,

trie measurements were

carried _out

only

_up to 0.7

À~l

_in

sinÙ/À.

This _goes

along

with _a limitation in atom

position

resolution wuich does _not allow _us to

distinguisu quantitatively

between these two models.

Tuis

problem

_was

already

found [4] ⁱⁿ relation witu tue

study

of tue _room temperature

phase

in trie TDP. Nevertheless,

comparing

to otuer compounds of this

farnily,

it _seems

likely

for

H(D)

atoms to occupy tue

split positions.

766 JOURNAL DE PHYSIQUE I N°7

~e ~ .

c a

ee

ai ai

~ e

D2

~ ~

e~ e e

a) b)

Fig. l. Trie orthorhombic structure model of trie high temperature phase for DTDP: a) alolrg trie a-axis;

b)

along trie _c-axis.

Table II. Fractional atomic coordinates

for

TDP and DTDP. Estimated standard deuiations

are giuen in

parentheses.

TDP DTDP

Atom xla

y/b

z/c xla

y/b

z/c

TLI

0

0.1279(2)

1/4 0

0.1277(1) 1/4

Pi 0

0.3749(3)

1/4 0

0.3748(2)

1/4

Oj 0.1283(8) 0.4355(3) 0.0806(6) 0.1339(6) Ù.4344(2) 0.0830(5) 02 -0.2486(7) 0.3129(2) 0.1706(5) -0.2451(4) 0.3126(2) 0.1668(3)

Hi(Di)

0 1/2 0 0 1/2 0

H2(D2)

0

0.8151(5)

1/4 0

0.8148(2)

1/4

Table III. Anisotropic ^thermal parameters

(in10~~

À~

): a) for

TDF and

b) for

DTDP.

a)

Atom Ujj U22 U33 U23 U13 U12

T1j 4.8(2) 5.6(1) 4.9(1) 0 -lA(2) 0

Pi 3.0(2) 3.5(2) 3.3(2) 0 0.8(2) 0

Oj 6.1(2) 7.3(2) 8.3(2) 4.5(2) 4.6(2) 3.1(2)

02 3.0(1)

6.4(2)

5.1(1) -2.3(1) -0.5(2) 0.9(1)

Hi 5.7(4)

9.7(6)

6.7(4) -2.1(5) 2.8(5) -1.8(6)

H2 7.5(5)

6.2(4)

5.4(3) 0 0.9(5) 0

b)

Atom Ujj U22 U33 U23 Uj3 U12

T1j

4.9(1) 5.5(1)

5.4(1) 0

-1.39(9)

0

Pi 3.0(1) 3.3(1) 3.4(1) 0 0.7(1) 0

OI 6.9(1)

6.7(1)

8.8(2) 4.6(1) 4.5(1) 3.2(1)

02 3.1(1) 6.2(1) 5.1(1) -2.07(8) -0.47(9) 0.45(7)

Dl 7.7(2) 8.6(2) 8.7(2) -3.3(2) 4.4(2) -3.4(2)

D2 10.5(2) 5.2(2) 5.7(1) 0 2.3(2) 0

Table IV. O-O and

O-H(D)

distances

(in À) for

trie two

dijferent

types

of hydrogen (deu- terium)

bonds, _as Weil _as P-O distances.

TDp uTup

P-Dl 1.524(3) 1.517(2)

P-02

1.528(4) 1.527(2)

Oi-llj(Di) 1.216(3) 1.245(2) 02"H2(D2) 1.253(3) 1.278(2)

O

i-O

_1> 2.43

1(6) 2.491(4)

02-02' 2.505(6) 2.556(3)

3. Phase Transitions in trie

TDP/DTDP System

As

presented

in the introduction _we expect to

observe,

_on

cooling

to _room temperature, different types ^of

phase

transitions for TDP and DTDP. In the _case of TDP ai l~ the

change

from

orthorhombic to monoclimc symmetry is

only

visible _m the

slight

deviation _m the

angle

o

from 90°. The TDP ferroelastic _room temperature cell parameters were: a =

4.525(3) À,

b =

14.33(1) À,

_c

=

6.526(6) À,

_a

=

91.76(7)°,

which _{are m} agreement ^{with reference} [4].

Nevertheless, ^{for DTDP} at

T)

_{a new} series of reflections with

(h/2 k/2

_{1) occurs,}

corresponding

to a C-centred lattice where the c-axis tums eut to be the monoclinic axis. This lattice _was

768 JOURNAL DE PHYSIQUE I N°7

therefore transformed into _a primitive one

by

the relation: a*

= a*

/2+b* /2,

b*

=

b*,

c*

= c*,

where

(a*, b*,

^c* are the vectors of the

reciprocal

lattice of the orthorhombic

high

temperature

phase

and

(a*, b*, c*)

of the _room temperature monoclimc

phase.

For this

setting

^the _space

group of the monoclinic

phase

for DTDP is

P2116.

The lattice parameters _are: a =

9.070(6 À,

b

=

15.00(1) À,

_c

=

6.574(5) ^À

and ₁₌

106.92(5)°

with Z

= 8. SO in the _case of DTDP the

volume of the unit cell has

doubled,

_{at room} temperature the a-axis

being

^the double of the a-axis of the orthorhombic

phase,

and the b-axis

being

the

diagonal

of the orthorhombic _a- and b-axes. The c-axis remains the _same. Two different monodmic _{axes are} thus

surprisingly

observed for the _room temperature

phase

^of TDP and DTDP. More detailed _neutron diffraction data of the _room temperature

phase

of the DTDP will be

published

elsewhere (14].

From this structural

study

_we have _seen that for DTDP at

Tf,

the transition from the orthorhombic to the monodinic

phase

is induced

by

_an order parameter which

belongs

to the double

degenerated SI

_(~) irreducible representation at the S point

(k

=

(1/21/20)

of the Brillouin Zone

(BZ)

of the _{space group} Pbcn, the monoclinic axis

being

^{the c-axis} of the parent

phase.

For TDP, the transition ai Ti is induced

by

_an order parameter ^which

belongs

to the irreducible

representation

_B3g ai the G

point

of

BZ,

and the monoclinic _axis is the a-axis of the parent

phase. Moreover, comparing

to the structures ai room temperature

phase,

at Ti in TDP

only hydrogen

atoms

designated

_as H2 get

ordered,

whereas below

T)

for DTDP bath

deuterium atoms _are ordered. In the _case of TDP it is necessary ^to decrease the temperature below

T)

= 230 K to induce the

ordenng

of the second kind of

hydrogen

atoms,

Hi

_(15]. This confirms that for the

TDP/DTDP

_system deuteration has effects _net

only

_in the ferroelectric

transition temperature, T~, ^{but aise} in the structural

phase

_sequence. Further

experiments

_are

m progress to determine the structure of TDP in ils antiferroelectric

phase.

Acknowledgments

We would like to thank M. Godard for _prepanng the

samples

of DTDP. We aise thank

Ministry

of Education of the

Basque Country (Spain)

for the support ^of a Research Grant for _one of _us

(S.R.).

References

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