The proton spin lattice relaxation times and Knight shifts were measured in f.c.c. (delta-phase) and f.c.t. (epsilon-phase) ZrH/sub x/ for 1.5  or = to x  or = to 2.0. Both parameters indicate that N(E/sub F/) is very dependent upon hydrogen content with a maximum occurring at ZrH1 83. This behavior is ascribed to modifications in N(E/sub F/) through a fcc/fct distortion in ZrH/sub x/ associated with a Jahn-Teller effect

Attalla, A.

Bowman, R. C., Jr.

Craft, B. D.

Rhim, W. K.

Venturini, E. L.

English

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A. Attalla*, R. C. Bc,wman, Jr.*, B. D. Craft*,
E. L. V ,n*_urini**, and W.-K. Rhim***
*Monsanto Research Corporation-Mound
Miamisburg, Ohio 45342
PRO WN NMR STUDIES OF THE ELECTRONIC
STRUCTURE OF ZrHx
**Sandia National Laboratories
Albuquerque, New Mexico 87185
*'*Jet Propulsion Laboratory
California Institute of Technology
Pasadena, California 91103
ABSTRACT
The proton spin-lattice relaxation tunes and Knight shifts have
been measured in f.c.c. (6-phase) and f.c.t. (6-phase) ZrH for 1.5
< x < 2.0. Both parameters indicate that N(EF ) is very dependent up-
on hydrogen content with a maximum occurring at ZrH I _ 83 . This be-
havior is ascribed to modifications in N(E F ) through a fcc-fct dis-
tortioh in ZrH a.:sociated with a Jahn-Teller effect.
X
The electronic properties of tha no.i-stoichiometric dihydrides
of the IVB metals Ti, Zr, and Hf have been the subjects of numerous
theoretical )-3 and experimental - ' 0 studies. Much of this interest
has focused on the tempe.ature and composition dependent fcc to fct
phase transition that has been associated' 2,4 with a Jahn-Teller
type mechanism, as well as the more general problem of the character
of the metal-hydro gen bonds. 115-8 Although there have been several
recent nuclear magnetic resonance (NMR) studies related to the elec-
tronic structure of TiHI , 6-8
 only limited NMR results 9 ' 10 eddress the
electronic properties of 6-phase (fcc) and e-phase (fct) ZrH . In
the present work, the temperature and composition dependences cf the
proton spin-lattice relaxation times (T1) ana Knight shifts (o ) have
been measured in high-purity polycrystalline ZrH for 1.5 < x 2.0.
These parameters, which are related l to the d.ansities of electron
HIS W.DMfil Is llMl!S"	
)
states N(EF) at the Fermi tuiergy EF , show the dominance of the care-
polarization hyperfine interaction with the Zr d-electrons and yittd
a wAximm in N(E ) near x - 1.83. These observations support the
Jahn-Teller mechanism'" for the tetragonal distortion and are con-
sistent with recent APW band-theory calculations  of fcc ZrKi and
photaemiasion spectra l for ZrH .
The ZrH samples were prepared by direct reactions between puri-
•fied Hz gas and zone-refinpd Zr foils (Materials Research Corporation -
NARZ grade). The ZrHx foils were ground under an argon atmosphere to
produce powders that were sealed in evacuated glass tubes. The Ti
values were obtained by the standard inversion-recovery method at the
proton frequency of 34.5 MHz. The previously describede zero-cross-
-I ng method produced the OK values, which are relative to an external
reference of tetramethylsilane, for a resonance frequency of 56.4
MHz. The a  values have a precision of t2 ppm while the exponential
Ti recoveries yielded Ti with a precision of ±33. The T1 values were
waasured over the temperature range 100 K to 300 K and the v values
were obtained between 170 R and 310 K, as shown in Figs. l and 2, re-
spectively. There was no unusual temperature behavior for either para-
meter. Below 310 K, the proton T1 values are assumed to be dominated
by the conductjon elect ron component Tie as found previously. 9110
The compositio., Lnd temperature behavior of the proton a and
(Tie T) ;4 parameters for ZrH are summarized in Fig. 3 where T 
x is the
absolute temperature. The major feature for (Tie T) -'^ is an increase
above .x = 1.65 to reach a maximum near x = 1.83 before decreasing
smoothly up to x = 2.00. (Tie T) -11 exhibits the largest temperature
dependence for 1.80 < x < 1.85 (i.e., at the peak). Similar behavior
is noted for the proton a  parameters; however, neither the composi-
tion nor temperature dependence of 
'UK is as large as seen for (Tie T)' .
The proton Tie and QK parameters are related to the electronic
structure of a metal hydride through the hyperfine fields ll produced
at the proton sites. Since the d-electron orbital term 11 appear to
make insignificant contributions to the proton parameters in several
metal hydrides, 7'8 the a  and Tie parameters can be separated as
G  2VB [Hh f (s) Ns (EF) + Hhf (d) Nd (EF) ]
	
(1)
_ r 
4n YH _B{[ hf ( s ) Ns (EF) ] 2 + [ hf (d ) Nd (EF ) ] 2q) .	 (2)T le
Here, Hnf (s) and Hhf (d) are the hyperfine fields for the Fermi con-
tact interaction with unpaired s-electrons and transferred "core"
polarization from the s-d exchange with metal d-states, respectively;
Us is the Bohr magnetron, Y. is the proton gyromagnetic moment; ks 2
is the Boltzma4 # s constant; and the reduction factor q - 1/3[f(t29)]
+ 1/2[1-f(t2g)] in d cubic structure where f(t2y) is the fractional
76
6
4
a
3
2
o ZrH,,a
O ZrH,,,,
o ZrH,,w
v ZrH, ,,
o ZrHz,00
G
O
°
°	 o °
0
1
080	 120
	 160
	 200
	
240
	 280	 320
TEMPERATURE (K)
Fig. 1. Proton T i
 for ZrHx
 at VH
 = 34.5 MHz.
r
i
Fll
-7v
•	 0 ZrH,,,,
	
-60	 D ZrH,,,,,
v ZrH,-,o,
a ZrH2."
-5p
	
:E -40	 -z^-d
a
d -30
-20--
	
.10	 _
0
160 180 200 220 240 260 280 300 320
TEMPERATURE (K)
Fig. 2. Proton Knight shifts for ZrH. measured by multiple-pulse
zero-cross technique at 56.4 MHz.
70
	
.60
65 A 300 K	 w
O 196 K	 (T,.n'"
A 115 K	 •
so -
♦ 169 KoK
S • 306 K
s 53
a-
w
o O
Tv
:r G
Y a
45	 O p p O
O O
	
O
a
a ^ a
40- 1
a + di d(fcc)	 6 +L 1 	 E&O
.50
•30
QCL
Y
.20 c
-10
0
35 1	 1	 1 1	 1	 1	 1+10
1.4	 1.5	 1.6	 1.7	 1.8	 1.9	 2.0
HYDROGEN STOICHIOMETRY, ZrHx (moles Fi)
Fig. 3. Composition dependence of proton (T 1,T) -3i and QK for ZrHx
character of the t29 d-orbitals at EF . The Hhf (s) is always positive,
but Hhf (d) is usually negative. 11 Thus, although the contact and
core polarization terms are additive for (T 1i T) -34, a large cancella-
tion can occur in UK.
The negative proton aK values in Fig. 2 clearly indicate that
the.core-polarization term exceeds the contact term in 6-ZrHx and
e-ZrHx , which implies- Nd (Ef ) X',Nf (EF ) in these hydrides. Similar con-
clusions have been made for several other hydrides including y-TiHx,7
Tit-yVyHx , 8 TiCuHx '12 and TiCr 2 Hx ; 13 hence, a relatively large N d (EF )
seems to be a general property of the hydride phases formed by group
IVS metals and of at least some alloys containing Ti. A more detailed
analysis of the proton aK and (T1eT)-^ values for ZrHx suggests sub-
stantial s-electron contact (or, perhaps, some orbital) contribution
in 6-phase ZrH1.60 while the core-polarization seems to be the exclu-
sive hyperfine interaction in a-phase ZrHx.
111
The composition dependence of (T1,T)-h and QK in Fig. 3 is con-
sistent with the available -nand theory c:4lculations for TiH2 , 1 ' 2 and
ZZR2, 3 as well as the Jahn-'seller mechanism 2i4 for the fcc-fct transi-
tion. Briefly, Nd (EF ) is apparently constant in fcc S-ZrHk (x < 1.65)
as the 4 level moves upward with x, but adding more hydrogen abovQ
1.65 causes EF to enter the band region where Nd (EF ) increases rapidly
until x 2:1.83. However, because of the electronic instability of a
large Nd (EF ) the tetragonal distortion continually increases, which
will tend to reduce the Nd (EF ) value as x becomes larger. The com-
petition between Nd (EF ) increasing with more hydrogen and decreasing
with the Jahn-Teller tetragonal distortion produces the (T1.T)-h
maximum near x z 1.83. This general behavior corresponds to changes
seen in the photoemission spectra  of ZrH X as hydrogen content varies..
A more detailed discussion of the electronic structure and proton
parameters will be published elsewhere.
ACKNOWLEDGEMENTS
This work was partially supported by the Division of Chemical
Sciences, U.S. DOE, and the Caltech's President's Fund. Mound is
operated by Monsanto Research Corporation for the U.S. DOE under Con-
tract No. DE-AC04-76-DPO0053. Sandia National Laboratories are sup-
ported by the U.S. DOE under Contract No. DE-AC04-76-DPOO789. JPL
is operated for the National Aeronautics and Space Administration
under Grant No. NAS- 7-100.
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e
17	
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T.
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Proton NMR studies of the electronic structure of ZrH/sub x/

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