Deuteration-induced structural phase transitions in some hydrogen-bonded crystals

Journal of

MOLECULAR

STRUCTURE ELSEVIER

Journal of Molecular Structure 378 (1996) 17-27

Deuteration-induced structural phase transitions in some hydrogen-bonded crystals 1 M i z u h i k o I c h i k a w a a'*, T a k a s u k e

Matsuo b

aDivision of Physics, Graduate School of Science, Hokkaido University, Sapporo 060, Japan bDepartment of Chemistry, Faculty of Science, Osaka University, Toyonaka 560, Japan

Received 21 July 1995;accepted 30 August 1995

Abstract

A review is given, mainly based on heat capacity measurements, of the deuteration-induced structural phase transitions recently found in some rather diverse types of hydrogen-bonded crystals: i.e. M3H(XO4)2-type crystals with strong hydrogen bonds, 9-hydroxyphenalenone derivatives with strong intramolecular hydrogen bonds, and (NH4)EMXr-type crystals with weak asymmetric hydrogen bonds. For M3H(XO4)E-type crystals and 9-hydroxyphenalenone derivatives, the hydrogen-bond distance seems to be one of the determinant parameters, while for (NH4)EMXr-type crystals it is as yet unclear. However, some common characteristics of the heat capacity behavior are noted: (1) the magnitude of the transition entropy is compatible with order-disorder-type phase transitions, and (2) unusually the values of the heat capacity of the undeuterated compound are larger than those of the deuterated compound at the lowest temperature range. Keywords: Isotope effect;Heat capacity measurement; Phase transition; Hydrogen bonding

1. Introduction In some of hydrogen-bonded ferroelectrics and related materials, it is well known that the substitution of deuterium for hydrogen leads to a drastic upwards shift of the transition temperature Tc (or inversely, the replacement of deuterium by hydrogen in the deuterated compound brings a remarkable downward shift of To) [1-3]. A downward shift of the transition temperature can also be * Corresponding author. t Presentedat the SummerSchool on Isotope Effectsas Tools in Basic and Environmental Research, Roskilde, Denmark, 24-28 June 1995.

achieved by applying pressure; the phase transition disappears under sufficiently high pressure in some crystals like KH2PO4 [4]. More recently, another type of remarkable isotope effect has been found in K3H(SO4)2, Rb3H(SO4)2, and Rb3H(SeO4) 2 of MaH(XO4)2-type crystals: a new phase appears on deuteration even at atmospheric pressure [5,6] (deuteration-induced phase transition). This phenomenon is quantum ferroelectricity induced by deuteration at ambient pressure [7]. Subsequently, the same phenomenon was recognized in (NH4)EPdCI 6 [8] and quite recently also in 5bromo-9-hydroxyphenalenone [9]. The number of the compounds exhibiting the same behavior has been increasing within each type of crystal. We

0022-2860/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-2860(95)09144-0

18

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27

therefore can say that the deuteration-induced structural phase transitions are seen in rather diverse types of hydrogen-bonded crystals: i.e. M3H(XOa)2-type crystals with a strong intermolecular hydrogen bond connecting two XO4 ions, 9-hydroxyphenalenone derivatives with strong intramolecular hydrogen bonds, and (NH4)2MX6-type crystals with weak asymmetric hydrogen bonds. However, it seems natural to consider that this phenomenon has not an accidental origin specific to a compound, but has a common origin to all these compounds. In this paper, the deuteration-induced phase transition is reviewed, especially by focusing on the results of heat capacity measurements and supplementarily by referring to structural and dielectric measurements. Although many of the compounds belonging to these types exhibit more than two transitions (successive transitions), we mainly confine our discussion to the transitions at the lowest-temperature region, below room temperature. Part of this topic is described in Refs. [10,11]. Structural and dielectric characteristics of the compounds are discussed briefly in section 2, the behavior of the heat capacity in section 3, and some remarks are made in section 4.

2. Structural and dielectric characteristics

M2 J~k

0-3

01 "~X M1 0

~ ( ~ a

Fig. 1. The b-axis projection of the crystal structure of M3H(XO4)2-typecrystals (M = K, Rb; H = H, D; X = S, Se) with space group A2/a. M(1) and M(2) are crystallographically independent.

2.1. M3H(XO4)2-type crystals The crystal structure of M3H(XO4)2-type crystals (M = K, Rb; H = H, D; X = S, Se) with space group A2/a is given in Fig. 1. The characteristics of this structure are (1) the proton is located at the center of symmetry or in two disordered positions around it (crystallographically symmetric hydrogen bond), (2) this strong hydrogen bond connects two XO4 ions, forming a dimer, (3) the dimer is surrounded by two kinds of alkaline metals, but is not hydrogen bonded to any other dimers at all, being "isolated" from the viewpoint of the hydrogen-bond network (zero-dimensional hydrogenbond network) [12]. The member crystals, in which most are isomorphous with space group A2/a, except for Cs3H(SeO4)2 which has space group C2/m, are classified into groups, i.e. the

compounds in which the protonated compound has no transition, but the deuterated one has a transition at finite temperature, and the compounds in which both the deuterated and the undeuterated compounds have the transitions. These are summarized in Table l(a). The temperature dependence of the dielectric constants of the member crystals is reminiscent of an antiferroelectric phase transition, although direct observation of the hysteresis loop has not yet been successful. As an example, the case of Rb3Hl_xDx(SeO4)2 [13] is shown in Fig. 2 (as for the dielectric constants in other member crystals, see references cited in Ref. [10]). Effects of hydrostatic pressure on the phase transition have been measured on K3H(SO4)2 and K3D(SO4) 2 [14]. The transition temperature for K3D(SO4) 2 was

M. lchikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27 Table 1 Transition temperatures in the members of the crystal families related to the deuteration-induced phase transitions a

19

Rb3Hl_xDx(Se04) 2

(a) M3H(XO4)2-type crystals M site

K Rb

X = S H

D

no no

84 K 71 K

5

X = Se 0 K Rb Cs

20 K no [20] 50 K

73 K [70% D) 95.4 K [20] 180 K

(b) 9-Hydroxyphenalenone derivatives

-5

5-site

H

D

Br CH 3

no [9,24] 41 K [9]

21.5 K and 33.9 K [24] 44 K [9]

0

0

I

1

50

I00

150

TEMPERATURE (K)

(c) (NH4)2MX6-type crystals X = CI M site

H

Sn Pd Pt Pb Te Se

no no no no no 24

D [34] [25] [27] [29] [30-32,39] K [35]

no [34] 30.2 K [26] 27.2 K [28] 38.4 K [29] 32 K and 48 K [30-33] 48 K [35]

a Unless references are given, see references cited in Ref. [10].

found to rise with pressure (dTc/dp = 8 K GPa-1), making a sharp contrast with other hydrogenbonded crystals exhibiting large isotope effects where much larger negative pressure coefficients of Tc are commonly observed [15]. However, no significant pressure effect was observed for K3H(SO4) 2. The microscopic origin of the pressure effect is not fully interpreted yet. In these M3H(XO4)2-type crystals, the linear relation between hydrogen-bond distance O---O and Tc seems to hold among the members with finite T~, as in the cases of one-dimensional CsH2POa-type crystals, two-dimensional squaric acid, three-dimensional KH2POa-type crystals

Fig. 2. Temperature dependence of the difference of the dielectric constant from the extracted value to 0 K, Ae = e(T) - e(0), for Rb3H~_xDx(SeO4)2 with different deuterium contents at a frequency of 100 kHz. The vertical arrows indicate the transition temperature Tc [13]. Reproduced with the permission of the Physical Society of Japan.

[10,11,16]. Therefore, the observed facts are compatible with the hydrogen-bond distance O - . . O (or more directly related hydrogen-bond parameters like proton-proton (deuteron-deuteron) site separation) being the determinant parameter in deuteration-induced phase transitions. 2.2. 9-Hydroxyphenalenone derivatives

Among 9-hydroxyphenalenone derivatives, the 5-methyl and 5-bromo derivatives [9] are similar to those in MaH(XO4)2-type crystals. The crystal structures of 5-methyl-9-hydroxyphenalenone and 5-bromo-9-hydroxyphenalenone are shown in Figs. 3(a) and 3(b). The characteristic feature of the hydrogen-bond system in this group is their intramolecular hydrogen bonds, in contrast to the intermolecular hydrogen bonds in the M3H(XO4)2-type crystals. The 5-methyl and the 5-bromo derivatives

20

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27

---j

Fig. 3. Crystal structures of 5-methyl-9-hydroxyphenalenone (a) and 5-bromo-9-hydroxyphenalenone (b). Methyl hydrogen atoms in (a) are disordered and not located. Hydroxyl hydrogen atoms are omitted. Reproduced from Ref. [9] with permission.

are reported to be isomorphous with space group Cc [9]. The hydrogen-bond distance O-.. O in the 5-methyl and the 5-bromo derivative is 2.512(3) .~, and 2.49(1) A, respectively. These values are compatible with having proton disorder in the hydrogen bond and the range of the hydrogenbond distance exhibiting a large isotope effect [17,18]. The 5-methyl derivative behaves like the M3H(XOa)2-type members with finite Tc in both deuterated and undeuterated compounds: i.e. (1) it

exhibits a phase transition at 41 K, (2) the transition temperature shifts upwards on deuteration (44 K), and (3) the temperature dependence of the dielectric constants is reminiscent of an antiferroelectric one [9] (see Fig. 4(a)). However, the 5-bromo derivative behaves like the members with no phase transition in the undeuterated compounds, but with phase transition in the deuterated ones: i.e. (1) the dielectric constants increase with decreasing temperature and become fiat below around 20 K and (2) a new phase transition appears at 36 K on deuteration (see Fig. 4(b)) [9]. We can say, therefore, for this type of crystal that the hydrogenbond parameters may be crucial for the occurrence of deuteration-induced phase transition, although the data are still lacking further elucidation. The relevant data are summarized in Table l(b).

2.3. (NH4)2MX6-type crystals (NH4)2MX6-type crystals, where M = Pt, Pd, Sn, Pb, Te, Se, and X = CI, Br, are simple ionic crystals with antifluorite structure of the K2PtCl6-type, and have a face-centered cubic "

(a)

N •~

"~.

u

r

0

50 100 150 Temperature (K)

, ~ - - - , ~~- - - - ~- - - - ! Temperature (K)

Fig. 4. Temperature dependence of the dielectric constants measured at 10 kHz: 5-methyl-9-hydroxyphenalenone (a) and 5-bromo-9-hydroxyphenalenone (b). Inset in (b) shows the dielectric constants for a 50% deuterated sample. Reproduced from Ref. [9] with permission.

.~"

-

-

w[

Ib'~

Fig. 5. The crystal structure of (NH4)2MX6-type crystals. Reproduced from Ref. [19] with permission.

21

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27

lattice with space group Fm3m. The crystal structure is given in Fig. 5. Depending on chemical composition M and X, some members of this family undergo structural phase transitions and change into a phase of lower symmetry, while other members show no indication of transition. In connection with M3H(XOa)2-type crystals and 9hydroxyphenalenone derivatives, we will hereafter focus our attention only on the members having transitions in the deuterated salts in the low temperature region, but having either no transition or transitions in the corresponding protonated salts. In view of hydrogen bonding, (NH4)zMX6-type crystals obviously differ from M3H(XOa)2-type crystals and 9-hydroxyphenalenone derivatives. Hydrogen bonds in this type of crystal are weak and chemically asymmetric. The hydrogen-bond distance N - H . . . X is, therefore, unlikely to be the crucial parameter to induce transition in the deuterated compound. In these compounds the barrier to N H 4 rotation is considered to be low ( ~ 500 K). The rotation of NHa(ND4) ions around

a 3-fold axis appears to be an important factor for the occurrence of the isotope-dependent phase transitions [19]. The key structural parameter to interpreting the occurrence of the transition or the magnitude of the transition temperature is as yet unclear. The precise neutron structural studies are thus highly desired to reveal these points. Dielectric behavior seems not yet to be known for these member crystals. It would be interesting to know if the temperature dependence of the dielectric constants shows a c o m m o n character, as described in M3H(XO4)2-type crystals and 9hydroxyphenalenone derivatives.

3. The behaviour of the heat capacity 3.1. M3H(XO4)2-type crystals

The

low-temperature

25

L

0.8 I

I

I

heat

I

capacities

of

h ~

I

I

I

q

I

300

20 • •

0.4 "7 © "7

o

° o • o

o

&o %o

15

"7

200

,2

o

o

'7

0.0

2

0

4

g~

6

" - l0

,o

@

4° • •

100

o o

i o ~ o~° 5

•

o°

eeo o ,lto

o

je • o °

~

00

i

h

I

100

I

200

0

I

300

T/K Fig. 6. The heat capacity of Rb3H(SeO4)2 (O) and Rb3D(SeO4)2 (O). The curve for Rb3H(SeO4)2 is shifted upwards by 50 J K -I tool-1 for clarity.

~ ° ° °

5

I

I

10

15

20

T/K Fig. 7. The low-temperatureheat capacity of Rb3H(SeO4)2 and Rb3D(SeO4)2. Filled circles and triangles denote the Rb3H(SeO4)2 data and open circles and triangles the Rb3D(SeO4)2 data.

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27

22 t

J

extended to lower temperature (from 2 to 20 K), revealed that the heat capacities of Rb3H(SeO4)2 are larger than those of Rb3D(SeO4)2 in the temperature range 2.9-38 K (Figs. 7 and 8) [21], in contrast to usual cases. One would expect a larger heat capacity for deuterated compounds than for protonated compounds, because the larger mass of the deuteron gives more densely distributed energy levels of molecular motion. The excess heat capacity of Rb3H(SeO4)2 reached a maximum at 1.7 J K -1 mol -l at 20 K and an analysis in terms of Shottky heat capacity function was attempted. The low-temperature limiting behavior of the excess heat capacity was reproduced by a twolevel Shottky scheme whose excited energy level is attributed according to a Gaussian function centered at (52.5 4- 0.3)R K with the full width at half maximum of (29.9 i 0.8)R K [21]. The lowtemperature heat capacity of the corresponding sulfates, Rb3H(SO4) 2 and Rb3D(SO4)2, from 12 to 304 K, also behaves in a similar way to the selenates. Heat capacity measurements on a mixed crystal K3DI_xHx(SO4) 2 have also been reported [22,23].

i

3 o

G

<1

o

o

1

0 0

I

I

......................... I

I

10

I

t

20

I

I

30

i;_i

40

T/K Fig. 8. Excess heat capacity of Rb3H(SeO4)2 compared with Rb3D(SeO4)2. Circles and triangles denote the data in Refs. [20] and [21], respectively. A full line is the modified Shottky heat capacity. See text.

Rb3H(SeO4) 2 and Rb3D(SeO4) 2 from 12 to 304 K [20] are given in Fig. 6. Corresponding to the dielectric behavior (Fig. 2), no thermal anomaly is observed in Rb3H(SeO4)2, while a higher-order transition was found at 95.4 K with the transition entropy 4.0 J K -1 mol -l. Further measurement, 90

I

I

~o~ oo

6033.9 K $

..~

,o

80

~ ~°°

-

o, °

21.5 K ,l,

@ 30 -

O~x

xx

~,5~.°'x 8.." × 5-bromo-9-hydroxyphenalenone o#0

o 5-bromo-9-deuteroxyphenalenone

° o° o

0q

I

I

25

50

75

T/K Fig. 9. The low-temperature heat capacity of 5-bromo-9-hydroxyphenalenone and its deuterated analog. Circles and crosses denote the data o f the deuterated and undeuterated samples, respectively.

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27

deuteration-induced phase transition has been found [8]. The heat capacity of the protonated compound (NH4)zPdC16 has no anomaly from 5 K to 365 K [25]. However, the deuterated compound (ND4)2PdC16 has a A-type phase transition at 30.2 K, with the transition entropy AtrnS/R = 2.005 [26], which suggests that the transition is of an order-disorder type. Similarly, the heat capacity measurement of the Pt salt (NH4)EPtC16 shows no transition from 6 K to 348 K [27], but by deuteration a A-type transition is induced at 27.2 K, with a very high peak and the transition entropy AtrnS/R = 1.216, which is also characteristic of an order-disorder transition [28]. The Pb salt (NH4)zPbC16 also belongs to this category, as shown in Fig. 10. (NH4)zPbC16 and (ND4)zPbC16 undergo a A-type phase transition at 78.1 K and 80.7 K, respectively, but the deuterated salt exhibits another first-order transition at 38.4 K. The entropy change of this first-order transition is 6.5 J K -l mo1-1. Furthermore, the noticeable point is that the heat capacities at the lowest

3.2. 9-Hydroxyphenalenone derivatives The heat capacity of the protonated salt of the 5bromo derivative showed no anomaly, consistent with the dielectric constant measurements. The deuterated salt, however, showed two peaks at 21.5 K and 33.9 K in the heat capacity measurements [24], although these were recognized as one peak at around 36 K in the dielectric measurements [9]. The heat capacity of undeuterated and deuterated salts of the 5-bromo derivative is shown in Fig. 9. We can say that the case of the 5-bromo derivative corresponds to the case of (NH4)2TeC16 discussed in the next section.

3.3. (N H 4) 2M.Y6-type crystals Let us begin the discussion by taking a category of crystals in which the protonated compound has no transition, but the deuterated one has transitions. Among these members, the Pd compound (ND4)2PdCI 6 is the first in which the

T/K

50,

0

1O0 ,

,40

200

•

•

7 o E

oO .o

•

7x: 100

q30

°1[

O

o

'7--~ E 7 ,,I

.-)

eo •

0

0

e0 • 0

;° %

23

% I

15

,, (NH4)2PbCI 6 [ O (ND4)2PbCI 6

20'

'10

T~ K Fig. 10. The low-temperatureheat capacityof (NH4)2PbC16and its deuteratedanalog.

24

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27 I

I

200

÷÷

2_ o E "7 x,,' o

o.

f J

f +÷ f÷÷÷ ÷

"÷+÷ #

100 -

/

/ 0

I 40

I 80

120

T (K)

Fig. 11. The low-temperatureheat capacityof (NH4)2TeCI6and its deuteratedanalog. Open circlesand crosses denote the data of the deuterated and undeuterated samples, respectively. temperatures (13-22 K) are larger for the hydrogenous salt than for the deuterated analog, as in the case of Rb3H(SeO4) 2 [29]. Similarly to the case of (NH4)EPbCI6, the Te salt (NHg)2TeC16 has a phase transition at 88 K, for which the TeC16 ion seems to be responsible (Fig. 11). The characteristic feature of this compound is, however, that deuteration induces two further additional phase transitions at 32 K and 48 K in addition to the one at 87 K corresponding to the protonated compound [3033]. The lower transition has clear first-order nature, while the higher one, which starts immediately after the lower one, has essentially higher-order nature, but changes to a first-order transition accompanying only slight latent heat at the end of the transition. As shown in Fig. 12, an unusual behavior, that the heat capacity of the deuterated salt is smaller than that of the protonated salt, is also recognized below 32 K. In this compound, an unusual phase sequence (cubic-rhombohedralmonoclinic-tetragonal), revealed by powder neutron diffraction, is also noted [30,31]. The Sn compound (NH4)ESnC16 belongs to the second category of members in which neither (NH4)2SnC16, nor (ND4)ESnCI6 exhibits any transition down to l0 K [34]. However, the Se

compound (NH4)2SeC16 belongs to the third category. That is, the protonated compound exhibits a transition at 24 K and the deuterated analog has a transition at 48 K (Fig. 13). The transition entropy 100

I O

I

50

010

I

j

I

~,~

-

J

I 20

I 30

40

T/K Fig. 12. An inverse isotope effect of the heat capacity in (NH4)2TeCI6 and (ND4)ETeCI6. Open circles and crosses denote (NH4)2TeCI6 and (ND4)2TeCl6,respectively.

25

M. Ichikawa, T. Matsuo/Journal of Molecular Structure 378 (1996) 17-27 300

I

I

I

I

I

ooOOO c~°°°°¢~'~°°°°°6e:°°-

250

~ 15C

E 100-

-

0

I 50

I 100

I 150 TIK

1 200

I 250

300

Fig. 13. The low-temperature heat capacity of (NHa)2SeCI6 and its deuterated analog: (A) (NH4)2SeCI6, (B) (NDa)2SeCI6. The curve for (NHa)2SeC16 is shifted downwards by 50 J K -1 mo1-1 for clarity.

of the deuterated compound is considerably higher than that of the protonated one [35]. A neutron diffraction experiment on the deuterated compound was also carried out recently [36]. It has been found recently that a bromide compound behaves similarly to the corresponding chloride compounds, which anticipates that the deuteration-induced phase transition will be found in some other bromide compounds. The compounds discussed above are summarized in Table 1(c). As we have seen so far, (NHa)aMX6-type crystals are the most rich in variety; (NH4)2MX6-type crystals have a larger number of members and include all the types for the combination of undeuterated and deuterated crystals (i.e. without-without, without-with, and with-with transitions for H - D salts). M3H(XO4)2-type crystals and 9hydroxyphenalenone derivatives, however, have smaller numbers of members and only two types of combination are included in these systems (i.e. without-with, and with-with transitions). Some effort to interpret the physical quantities among

the members from a unified viewpoint were performed. A linear correlation between NH~- reorientation activation energy Ea and the lattice constant of the cubic phase a0 is noted. It was also pointed out that the tunnel frequency of reorienting NH~ions in tetrahedral surroundings correlates with a0 [37]. More recently, a correlation between a0 and Ea for hindered rotations about the 3-fold axis was discussed [38]. A unified, satisfactory interpretation, however, seem not yet to have been attained.

4. Some remarks

As we have seen so far, deuteration-induced structural phase transitions are a phenomenon appearing in rather diverse types of hydrogenbonded crystals. Nevertheless, some common thermal characteristics seem to exist: (1) the magnitude of the transition entropy is compatible with order-disorder-type phase transitions and (2) the values of the heat capacity of the undeuterated

26

M. Ichikawa, T. Matsuo/Journal o f Molecular Structure 378 (1996) 17-27

c o m p o u n d are l a r g e r t h a n those o f the d e u t e r a t e d c o m p o u n d at the lowest t e m p e r a t u r e range. F u r t h e r m o r e , systematic studies on the m e m b e r s in each g r o u p o f m a t e r i a l s seem to indicate t h a t the d i s a p p e a r a n c e o f the t r a n s i t i o n in the p r o t o n a t e d c o m p o u n d is a p h e n o m e n o n lying on a n e x t e n d e d line o f the d o w n w a r d shift o f T c b y a h y d r o g e n a t i o n o f the d e u t e r a t e d c o m p o u n d s . It m a y be n a t u r a l to r e g a r d this p h e n o m e n o n as h a v i n g a c o m m o n origin. T h e n the question is w h a t is the m o s t crucial p a r a m e t e r ? It is difficult to answer this at the present time, b u t the following p o i n t s m a y be w o r t h y o f r e m a r k . F r o m the p o i n t o f view o f the s t r u c t u r a l p a r a m e t e r s , as far as the h y d r o g e n - b o n d distances a n d angles are concerned, we can say t h a t the O . . - O distance seems to be at least one o f the d e t e r m i n a n t p a r a m e t e r s for the occurrence o f d e u t e r i u m - i n d u c e d p h a s e transitions for M a H ( X O 4 ) E - t y p e crystals a n d 9 - h y d r o x y p h e n a l e n o n e derivatives, b u t o b v i o u s l y this is n o t the case for the h y d r o g e n b o n d in (NH4)2MX6type crystals since the h y d r o g e n b o n d in this g r o u p is w e a k a n d a s y m m e t r i c . H o w e v e r , there still exists the possibility t h a t the p r o t o n - p r o t o n (or d e u t e r o n - d e u t e r o n ) distance o f the three p o s i t i o n s a r o u n d the 3-fold axis (see Fig. 5) p l a y s a crucial role. T h e c o m m o n local structural p a r a meter, which decides w h e t h e r a d e u t e r a t i o n i n d u c e d phase t r a n s i t i o n really t a k e s place o r n o t a n d is c o m p a t i b l e with all the o b s e r v a t i o n s in the three categories o f crystal, m a y be the i n t e r p r o t o n (or i n t e r d e u t e r o n ) distance. F r o m this v i e w p o i n t precise n e u t r o n s t r u c t u r a l studies are expected. In a n y case the origin o f the d e u t e r a t i o n - i n d u c e d p h a s e t r a n s i t i o n has to be e x p l a i n e d as a n effect o f the n u c l e a r m a s s via the q u a n t u m n a t u r e o f the m o l e c u l a r m o t i o n r a t h e r t h a n as a direct m a s s effect o f the m o l e c u l a r force.

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