Vibrational studies of structural phase transitions in partially ordered solids

Journal of Molecular Structure, 217 (1990) 35-49 Elsevier Science Publishers B.V., Amsterdam - Printed

VIBRATIONAL

Alexandre

STUDIES

OF STRUCTURAL

35 in The Netherlands

PHASE TRANSITIONS

IN PARTIALLY

ORDERED

SOLIDS

NOVAK

Laboratoire

de Spectrochimie

94320 Thiais

Infrarouge

et Raman, CNRS, 2 rue Henri Dunant,

(France)

ABSTRACT Some partially and amorphous

(dis)ordered

systems

solid have been studied

Examples

given are those of pseudo-spin

ammonium

hydrogen

superionic

selenate

NH4HSe04

to paraelectric,

phosphate

transformation

protonic

hydrates.

several

perfect

35H2P04, from

and low temperature

: antimonic acid and

and degree of structural

mechanisms

crystal

phase transitions

ferroelectric

conductors

The nature

and conductivity

between

and Raman spectroscopy.

proton glass Rho 65(NH4)o

undergoing

incommensurate,

phase and two other superionic zirconium

intermediate

by infrared

disorder,

the

are discussed.

INTRODUCTION

Among numerous problems

applications

of vibrational

those dealing with structural

phase transitions

interesting.

It turns out that a number

(dis)ordered

systems which are intermediate

amorphous

solids.

They include

orientationally encountered

disordered

in superionic

crystals,

nature phases.

The chosen

type. ammonium transitions 0022-2860/90/$03.50

examples

perfect

phases

to partially crystals

exhibiting

pseudo-spin

liquid crystals,

spectroscopic

of the transformation

hydrogen

appear particularly

and

perfect

proton

quasi-liquid

long

glasses, state as

and gels.

solids and the type of information and mechanism

to solid state

correspond

between

periodicity,

conductors

Here we wish to describe ordered

of phases

incommensurate

range order but no translational

spectroscopy

are proton

manifestations

of some partially

we can obtain about the order, between various

spin glasses

more or less ordered

of the Rbl_x(NH4)xH2P04

selenate

NH HSeO which undergoes several phase 4 4 from the high temperature superionic phase to the paraelectric,

0 1990 Elsevier

Science

Publishers

B.V.

36 incommensurate,

ferroelectric

and a low temperature

H2Sb4011. nH20 and crystal

hydrate

conductivity

mechanisms

PSEUDO-SPIN

GLASSES

The solid solution

a frustrated

existence

protonic

of ferroelectric

conductors

RbH2P04

in the high temperature

OH . ..O hydrogen

bonded

interactions.

of a minimum

relaxation

in superionic

antimonic

acid

in Zr(HP04)2.nH20.

Possible

are also discussed.

Rbl_x(NH4)xH2P04

which are isostructural

antiferroelectric

phase,

to-gel transition

and antiferroelectric paraelectric

system with competing

Slak et al.

NH4H2P04,

phase, represents

ferroelectric

and

have shown by NMR that the 87 dependence of the Rb spin-lattice

in the temperature

1

time Tl for x = 0.35 indicates a tremendous progressive slowing down -11 -7 set below 40 K not found in from 10 to 10

of the OH . ..O proton jump motion RbH2P04

crystal.

The occurence

Tl minimum

exhibits

the glassy

state

helium

of correlation

infrared

and Raman spectra

[2]. Th e most profound

temperature

for the paraelectric-ferroelectric

which

split in a number

the low-temperature

skeletal

but also of orientational In fact, low-frequency

Rbo.65(NH4)0.35H2P04 changes

solid solution

phosphate

information

the strength

practically ponding

rather

vibrations

disorder

bond associated

of ammonium

This spectroscopic disorder

crystal.

out much in

feature

is

of ammonium

from the vibrational bonds which

as those in pure crystals.

intrinsic

anharmonicity

and

and

OH...0 protons.

give rise to characteristic

of any kind but isan

and in

and probably

of pure RbH2P04

(several hundred wavenumbers)

with vibrational

components

at 20 K is smeared

and the type of OH... 0 hydrogen

ABC bands the width of which structural

than to disordered

the same in solid solution

OH stretching

and lattice-infrared

on the other hand, do not show any

orientational

which can be obtained

have been observed

: the room temperature

of a well-orderedmolecular

temperatures.

as being due to a frozen-in

tetrahedra

Another

spectra,

and NH4H2P04

not only in terms of ordering

ordering

and the lattice Raman spectrum

the same way as those at higher interpreted

bending

Raman spectra

at 20 K (Fig. 1) are characteristic

comparable

of

room and liquid

and well resolved

phase. This can be interpreted

of OH ...O protons

NH4H2P04

changes

in NH4H2P04

of narrow

phosphate

groups.

between

spectral

transition

phase shows very broad NH stretching,

concerns

of RbH2P04

and of Rb0.65(NH4)0.35 H 2PO 4 solid solution

Raman-bands

together with the

times characteristic

cl].

We have investigated crystals

of the line width broadening

a wide transition

property

spectra remain The corres-

broad and strong is not due to of a strong hydrogen

of the OH group and the particular

shapes of theOH... 0 potential curves in the fundamental

and excited states

bl.

37

Fig. 1. Low-frequency Raman spectra of pure RbH2P04 of Rb0.65(NH4)0.3gH2P04 solid solution at 20 K.

AMMONIUM

HYDROGEN

Ammonium

SELENATE,

hydrogen

and NH4H2P04

electric

selenate

(abbr. AHSe) has been extensively

B2 phase and via the (intermediate)

Pl and low temperature

of the deuterated

derivative

transforms

studied

[4-71

ND4DSe04

phase

to ferro-

(Fig. 2). The behaviour

(abbr. ADSe) is different

:

; stable phases I, II, VI and metastable

phase is orthorhombic

to the B2 paraelectric

phase over to the

incommensurable

non-ferroelectric

Fig. 2. Phase diagram of NH4HSe04 phases III', III", IV', V'.

the room temperature

and

NH4HSe04

and it has been shown that goes from the high temperature paraelectric

crystals

PZIZIZ1

phase on heating

and non-ferroelectric

and

[6]. Our study of AHSe and

38 ADSe as a function samples

of thermal and mechanical

[8], on the other hand, reveals

phase sequences

treatment

that there are essentially

in both AHSe and ADSe crystals

must be ascribed

to a delicate

and V') and stable

equilibrium

and that apparent

involving

of

the same divergencies

(III', III", IV'

metastable

; i.e. on cooling the room temperature

(I, II, VI) phases

phase II we can have either

of a large number

II-III '-IV'-V' or II-III"-VI

transition

sequence

(Fig. 2).

II-III'-IV'-V'

transitions

Paraelectric

phase II

Phase II is monoclinic

and belongs

to B2 space group with three formula

units in the unit cell [4]. Th e structure

consists

of infinite

chains of HSe04-

ions linked by strong OH...0 hydrogen

bonds

(Fig. 3). There are two non-

Fig. 3. Structure

phase

(II) of B2 symmetry

equivalent

selenate

0 . ..0 distances to NH

disorder

and orientational symmetry 2 rules observed

C2 and CL sites, respectively

hydrogen

spectroscopy

disorder

and thus two

chains are linked

The results

of x-ray

[9] show that there are three kinds

and selenate

is shown spectroscopically

same type of disordered

OH...0

for instance,

by external

C2 symmetry

ions. The statistical

by a non respect

protons,

out that in the crystals paralectric

the selection

but not by internal

phases

site nature

of the C2 selection

of Raman lines due to skeletal

modes [9]. It should be pointed

are followed

bonds.

HSe04-

bond giving rise to a statistical

of ammonium

by the polarization

and CsH 2PO 4r111,

[4].

: a proton disordered between two equivalent potential

of an OH . ..O hydrogen

of c

NH...0

[4 ] and vibrational

of structural

bending

groups occupying

of 0.256 and 0.252 nm. One dimensional

Ions with much weaker

diffrzltion

minima

of the paraelectric

stretching containing

of squaric

acid

and the [lo]

rules derived fromcrystalsymmetry modes.

The former see thus the

39 (average)

crystal

the latter

symmetry

see the instantaneous, this type of disorder

Moreover,

band broadening. causes

much in the same way as x-ray or n diffraction

The orientational

the broadening

is observed

necessarily usually

disorder,

of internal

for the paraelectric

decreasing

narrower,

is characteristic

The existence initially

increases

phase

smoothly

sample below

1131

with decreasing

phases of NH4HSe04

disorder

Finally,

molecular

of cations

and ND4DSe04

and anions

the Raman spectrum

is

of phase VI

crystal.

phase between 77

252 and 262 K in AHSe

Se NMR measurements

temperature

[5] has been confirmed

and at Tc = 251.5 K locks in

value k = l/3 of the ferroelectric

is more complicated

275 K exists

ions, [lo]. This

: the satellite wave vector along the c direction

into the rational

The phase situation

bands

III'

on the ground of

diffraction

discontinuously

temperature.

of an incommensurate

suggested

by neutron

of various

of a fully ordered

Incommensurate

of ammonium

to

phase of AHSe and ADSe as shown in Fig. 4. The

and the orientational

with decreasing

particularly

symmetry.

significantly

and even more so of external

Fig. 4. Low-frequency Raman spectra at different temperatures [9].

bands become

lower, molecular

does not contribute

while

phase IV'.

in ADSe, where among other things,

in a state of several modulations

the

of slightly different

lengths [9,13].

The vibrational limited

manifestation

to the low-frequency

of the incommensurate

Raman spectra

to be

: a ZZ(cc) polarized Raman scattering

of a single AHSe crystal

gives rise to a band at 90 cm

Rayleigh

(Fig. 5). The intensity

wing broadening

phase III' appears

-1

with a simultaneous

of both features

increases

40

l

-I

I

I

200

100

150

Fig. 5. Low-frequency Raman spectra zation as a function of temperature

rapidly

to reach a maximum

50

10 d/cm-'

of a NH4HSe04 [9].

in the incommensurate

single crystal

phase region.

in ZZ polari-

The 90 cm -' band

polarization in phase polarization in phase IV' but an a XX(aa) shows a uYY(bb) II. In the incommensurate phase III' the modulation along Z(c) axis perpendicular to the chain plane induces This result neutron

is consistent

diffraction

modulation

changes

X(a) direction

data

atomic displacements

with the absence [13]. The present

the induced polarization

important

characteristic

(AHSe/ADSe)

frequency

ratio

translational

play an important

Ferroelectric

axis

satellite

in the

thus suggest

from Y(b) direction

of the 90 cm

1.05 which

vibration

indicates

of ammonium

role in creating

that the

in phase IV' to

-1

band is its isotopic

that this band is due to

ions. Gliding

the incommensurate

of cations

could thus

phase.

phase IV'

The ferroelectric monocline

Raman results

in phase II.

Another

mainly

in the XY (chain) plane.

of longitudinal

B2 structure

phase

(Pl, Z = 3) results

spontaneous

(Fig. 3) [14]. R ecent neutron

polarization

from a small distortion being parallel

and x-ray diffractions

of the

to the b (chain)

results

[9,13]

show a

41

Another decrease

structural

information

of the OH stretching

concerns

frequency

0.. .O distance

a much more pronounced

OH...0 hydrogen

bonding.

at the IV'-V' transition

shortening

The sudden

(Fig. 6) shows

(about 10 pm) than for the

the use-0 - uSeOH frequency difference (AU) Simultaneously, -1 -1 . in phase IV' and m phase II to 73 and 67 cm (Fig. 6) from 84 cm

II-IV' transition. decreases

V', respectively. bond strength

This Av value can also be used to estimate

and increases

found for a number

with the increasing

of hydrogen

the OH...0 hydrogen

O...O distance

as it has been

LI51. Moreover, it can be in infinite chains since for

sulphates

and selenates

that HSeO - ions must still be associated 4 cyclic dimers much higher Av values are expected 1151. It can be concluded claimed

the IV'-V' transition rearrangement

VI-III"-11

is a first order transition

with a strong participation

which undergoes

of HSe04-

that

a structural

ions.

transition

Phase VI

The structure

of the room temperature

x-ray diffraction

phase of ADSe has been determined

[7]. The phase VI belongs

and the unit cell contains

four formula

to the orthorhombic

by

P.212121 symmetry

units. There are thus four equivalent

of HSeO - ions linked by an OH... 0 hydrogen bond of 0.2572 nm [7] 4 slightly weaker than those in phases II to V'. The phase VI is stable.'down to chains

liquid helium

temperature

is that of a completely

and its Raman spectrum,

ordered

crystal

VI of AHSe which can be obtained phases

even at higher

temperatures,

(Fig. 4). The same is true of the phase

by an appropriate

treatment

of either of the

II, III' or VI' [S].

VI-III"-11

transition

Calorimetric transition appears

[8] and neutron

of ADSe must have an intermediate

on heating

in which 1212121

the existence

structure

an important

ammonium

[9,13 ] results

show that the VI-II

incommensurate

phase

at 343 K (k < l/4). Further

and 360 K indicates

implies

diffraction

ions with respect

of a 2c superstructure

is embedded

rotation

cooling

and heating

(III") which between

300

(k = l/4) of B2 lattice

below 307K. In fact, the VI-II transition

of about 25“ of selenate

groups

to the chains and can be achieved

and gliding by a common

of super-

structure.

Raman spectra frequency

(Fig. 7) are consistent

Raman bands of a single crystal

their symmetry

species

with this interpretation. ADSe

using band polarization

The low

(phase VI) have been assigned and to the approximate

type of

to

42 3c superstructure

in both AHSe and ADSe crystals implying nine non equivalent 2 ions which has also been observed by D NMR [5]. Vibrational spectros-

selenate

copy appears

to be less sensitive

In the case of Se-OH stretching

as far as the band multiplicity

mode, -1

nents at 742, 745, 759 and 763 cm

for instance,

is concerned.

four single crystal

compo-

, instead of nine expected have been found

[91* The OH . ..O hydrogen bonds do not vary much in going from the phase II to III' and IV' as shown by small variation of the OH stretching frequency in the 300 to 120 K range

(Fig. 6) and similar

Fig. 6. OH stretching crystal as a function

difference

is observed

ferroelectric ammonium

in the low-frequency

tetrahedra

Low-temperature

phase V'

The crystalline

structure

and Raman spectra indicating amuch

in spiteof

the orientational

and better

The main

the bands of the

resolved

some ordering

indicating

of

(Fig. 4).

different

structural modifications.

higher number of narrowbands

the phaseV1

pattern.

Raman spectra where

of phase V' below

are considerably

stronger

spectral

and Se-O and Se-OH stretching frequencies of NH4HSe04 of temperature fortheV'-IV'-III'-II-I phase sequence [9].

phase are narrower

and selenate

general

The infrared

from those of the phase IV' The lattice region (Fig. 4) exhibits

which, however

the lower temperature.

100 K is not known.

, are still broader

This canbe

order in phase V' is not complete

than those

explained assuming that

even at 10 K.

of

43

ADS8

1 AIa m

LB

1 8P

lo-

c--,

“I

Fig. 7. Lattice Raman frequencies function of temperature [9].

on the ground of isotopic

300

200

100

motion

300

,,,” * II c,_ 400 T/K

of a single ND4DSeO4

frequency

shifts

crystal

[9]. It turns out that the

bands at 121 (A), 103 (A) and 67 cm -' (B3) are due to mainly vibrations

(phase VI) as a

ND4' translational

and those at 84 (B2), 66 (A), 61 (B2), 50 (B3), 46 (A) and 30 cm-l

(B3) to mainly

DSe04+ motions

(Fig. 7). The frequency versus temperature plot -1 frequencies are shows that the 121. 103, 84, 66 and 44 cm -1 are not. The modes sensitive while those at 67, 50, 46 and 30 cm

of the above modes temperature corresponding frequencies T

C

to the former are parallel decrease

= 340 K with

Furthermore,

mechanism

phase III" (Fig. 7). This behaviour

(AH = 667 J/mole)

of ammonium

of selenate

is supported

a and c crystallographic range.

transition

the two successive

(gliding) motions reorientation

to the b (chain) axis and their

even far from the transition

temperature

changing the slope of the curve in the interval corresponding

the incommensurate first order

continuously

has a considerable

transitions

ions parallel

ions and/or

indicates

correspond

to mainly

This explanation

by the fact that the frequencies axes are remarkably

displacive character. translational

to the chains followed

chains.

constant

to

that the VI-II

by a

of the transition

of the modes

parallel to

in the same temperature

44 II-I transition

The high-temperature unusually

ionic phases symmetry,

phase I of AHSe above 409 K is superionic

high conductivity

o = 10-20hm-1cm -I [16] similar

of CsHS04 and CsHSe04

however,

being close to the hexagonal

There is a great similarity melt and a considerable particular, bands,

respectively,

with the spectrum

stretching

are observed

is monoclinic

(P21/b) its

system [91.

of the Raman spectra

difference

in the skeletal

can be interpreted

[17]. The crystal

with an

to that in super-

and bending

of the phase I and of the of phase II (Fig. 9). In

region,

only three and two

instead of four and five in phase II. This

in terms of almost

C3v symmetry of selenate ion in phase I -1 state. The vSe0 - vSeOH splitting Au = 150 cm , is also similar -1 to that of the melt, Au = 147 cm , but much larger than that of the phase II -1 (83 cm ) indicating a conversion of infinite chains to open dimers and an

and in molten

appreciable

weakening

of OH...0 hydrogen

bonds

(Figs. 6 and 8). The same trend

Fig. 8. Raman spectra of the skeletal stretching region of NH4HSe04 at various temperatures from 300 to 500 K. The temperature increased during the a, b, c sequence and decreased during d, c sequence.

is shown by the high frequency absorption

(Fig. 6). Ammonium

shift and broadening

of the OH stretching

ions must also have a much higher

in phase I and in the melt since theNH

site symmetry

stretching band structure disappears

191.

45 In the region of lattice vibrations

of phase II (Fig. 4) is smeared

the spectra broadening disorder

and are characteristic as well as anions

tric relaxation

measurements

state since an analogous inelastic

neutron

of a plastic

while

mechanism between

neighbouring

480 cm

neutron

involving

selenate

by microwave

for instance,

dielec-

yields

an

a liquid (Fig. 9)

(P(o)) similar tothatof

:

’

scattering,

phase shows particularly

vibrations

structural

phase implying a rapid reorientation

(P(w)), of CsHS04

phase of CsHS04 has a low conductivity

in such quasi liquid

profoundly

show a serious

phase of CsHS04,

spectrum

inelastic

the superionic

and rotational

in

[18]. Such state is frequently called "quasi liquid"

160

the low-temperature

observed

out and there is a considerable

on given sites, also evidenced

superionic

scattering

Fig. 9. Low-frequency Zr(HP04)2.H20 [20].

all the structure

line. The above results

of the Rayleigh

of cations

practically

protonic

[19] and

and narrow bands

broad bands due to translational species

b91- The

conductivity

state of AHSe may involve both proton ions and hopping

of ammonium

transfer

ions and differs

from that in the room temperature

for several

orders of magnitude,

phase where the conductivity drops -1 -I o = 10m70hm cm The assumption of a

quasi liquid

state in the superionic

measurements

of the II-I transition

b4

phase is supported enthalpy

which

by the calorimetric

amounts

i.e. about the same as that of the I-melt transition

to about 42 k.J/mole,

b1.

Conclusion

In the phase sequence ammonium

and selenate

V'-IV'-III'-II-I

ions increases

in phase I. The participation at both ends of the sequence The VI-III'-11

the orientational

progressively

of HSe04- motions

appears,particularly

where OH . ..O hydrogen

firstordertransitionis

that froma

disorder

of both

to reach a quasi liquid state important

bonds are seriously fully ordered

modified.

to a disordered

46 state, has essentially

displacive

character

and consists

of gliding motions

ammonium ions along the chain (b) axis followed by a reorientationof

ANTIMONIC

ACID HYDRATE,

Previously conductors

discussed

AHSe belongs

conductors

contains

broad channels

suitable water

the absorption

and temperature. (i) OH groups

since

H 2 Sb 4 0 ll.3H20 is among the best

Infrared

The structure

spectra

of anti-

(0 < n < 3) show that the skeletal

structure,

does not change.

of the OH groups

is expected,

species

to Sb4011 skeleton,

narrow band at 3635 cm -1, (ii) water

protonic interesting

u3GoK = 2.10-30hm-lcm-1.

Three kinds of protonic

attached

which appears

for water molecules. content

and thus rigid framework

region, where

content.

with the conductivity

manic acid with different region,

to the class of anhydrous

may depend on the water

protonic

of

ions.

H2Sb40Ll.nH20

but there is also a class of hydrates

the conductivity

selenate

The high frequency

depends

strongly

can be distinguished

(Sb409(0H2)),

and (iii) oxonium

characterizd

[21]

on n

:

by the

ions H30. The latter can

be identified in the bending region at 1750 cm -' (dH30+) distinct from&H20 near -1 -1 while the OH stretching region between 3600 and 2800 cm appears more 1670 cm complicated

and can roughly be interpreted

3380

H,

3240

H'

assuming

,H 0 - H...O \H 2950

that oxonium

as follows

3530 3580

ion forms stronger

hydrogen

bonds than water.

can form shorter or longer H30 + (H20), chains which

nated water

rigid framework

channels.

can be directly

hydrogen

In addition,individual bonded

to the framework

oxonium

left since in the spectrum

of an isotopically

band

increases

(1680 cm-') intensity

1460 cm-') with decreasing

equilibrium

diluted

with respect

ions andwatermolecules

oxygen atoms.

+ the H30+ + H20 + H /Sb4011

At lower temperature,

Such proto-

can fill up the

sample

is shifted

to

(Fig. 10) 6HOD2+

to that of the 6HOD band

(at

temperature.

As far as the conductivity two parallel and proton appears

processes

transfer

:

mechanism is concerned, there are likely to exist + ions on different sites in the channel jumping of H30

within

less important

the water

because

0.42 to 0.47 eV for trihydrate. required

for H,O+ hopping

in

chains

of relatively

(Grotthuss

In fact higher values

6 alumina while

mechanism).

high activation

The latter

energy value of

E = 0.3-0.5 eV are a for the proton transfer E < 0.2eV. a

I

DO

I

I

3500

!

2500

3000

2000

Fig. 10. Infrared spectra of isotopically diluted D2Sb4011.2D20 at different temperatures [21].

CRYSTAL

TO GEL TRANSITION

considered

are relatively

and surface

conductor. properties,

[22]. The structural

well established

1500

(H/I;dO.1)

sample of

the rigid framework spectra

This compound

arrangement

can be

bands. The situation

such as Zr(HP04)2. nH 20 which

particularly

(Fig. 11) while

between

weak and the infrared

of the corresponding

in some other hydrates

for its ion-exchange materials

acid the interactions

as a superposition

complicated intrinsic

species

I

I

IN Zr(HP04)2.nH20

In the case of antimonic and protonic

I

cm-l

is more

is a mixed

has been known for a long time in the treatment

of irradiated

of Zr ions and phosphate

that of protonic

species between

groups

is

Zr(P04)2

Zr(HPO.J,.nH,O

Fig. 11. Left : diagrammatic representation of a (Zr(P04)2)n layer and of the "zeolithic" cavity between layers of the compounds dried between 30 and 200°C Right : infrared spectra of (a) amorphous gel, (b) optical transparent gel, (c) crystalline sample of Zr(HP04)2.nH20.

PI ;

48 layers is not. Half of the exchangeable oxonium

[22]. This material

ions

amorphous

gel with n = 6-12,

(ii) optically

n = 2 and (iii) small crystals

Infrared

spectra

and differences bending

bands.

crystallinity, nature

transparent

to occur as

monolithic

and crystalline

: (I)

gel with

samples are quite different

OH and H20 bands as well as skeletal

This shows that there is not only difference but also that a more substantial

occurs

are believed

in three main forms

with n = 1.

of amorphous

concern

hydrogens

can be prepared

stretching

and

in particle

rearrangement

size and

of a chemical

[20]. Spectrum

of crystalline sample exhibits (Fig. 11) narrow -1 due to more or less free water (on well bands near 3600, 3500 and 1600 cm -1 defined sites in the cavity) and broad strong bands near 3000 and 2300 cm 2attributed to strongly associated HP04 groups with an O... 0 distance close to 0.26 nm. Amorphous

samples,

but a broad distribution

on the other hand, do not show the same type of water

of hydrogen

bonded water.

Other protonic

species may

include HP0 2-, H30+, Zr-OH2 and Zr-OH groups which can give rise to numerous 4 bands in the OH bending region between 1800 and 1200 cm -I (Fig. 11).

Low frequency

neutron spectra (Fig. 9) show H30+ and H20 librational bands -1 and translational bands near 150 cm-l of a sample at room

near 660 and 480 cm temperature quasi

while

at 500 K the structureless

spectral

feature

is typical

of

liquid state.

Spectral

and calorimetric

can be represented

results

by the following

Zr(HP0 4 )2_x(H30)x(n-x)H20 humidity

and crystallinity.

is likely associated PO4 groups.

and a high proton rigid framework

that zirconium

where n and x values The transition

with defect

Such defects

suggest

"dissolves"

hydrate

formation

depend on temperature,

from crystalline

to amorphous

such as HP207 and proton

do not allow the formation

concentration

phosphate

formula:

leads to amorphous

of infinite gel state.

and forms a layer between

state

transfer

Zr(P04)2

In other wordsathe

liquid and solid.

-3 -1 -1 Conductivity varies from 6.10 ohm cm for an amorphous sample to -1 5.10-60hm-Icm for crystalline material while the conductivity of dehydrated -9 -1 -1 sample drops to 10 ohm cm . The conductivity depends considerably on the surface

adsorption

capacity

and thus on atmospheric

humidity.

The conductivity

mechanism in gel is similar to that in antimonic acid and consists of hopping + of H 0 and H 0 species as well as of proton transfer : the activation energy E

a

=30.3 eV aipears

a little

too high for a pure Grotthuss

on

layers

mechanism

[20].

49

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