in some  beyond 
 li - ion battery  concepts , li metal 
 is used as the anode , e.g. , in li / air , li / sulfur , and some redox flow 
 batteries .
li metal benefits from a high 
 theoretical specific capacity ( 3860 ma h g ) , low 
 density ( 0.59 g cm ) , and the lowest negative electrochemical 
 potential ( 3.04 v vs the standard hydrogen electrode ) leading 
 to high theoretical energy densities .
nevertheless , safety issues 
 related to the formation of li dendrites in cells with liquid electrolytes 
 has stymied commercialization of rechargeable high energy batteries 
 with li - metal anodes . because of these safety issues , there is renewed 
 interest in the use of solid electrolytes either in all solid - state 
 devices with li - metal anodes or to protect the li - metal anodes in 
 a hybrid system that also utilizes a liquid electrolytic solution .
garnets based on li7la3zr2o12 ( llzo ) , which show 
 high li - ion conductivities and excellent chemical and electrochemical 
 stability including apparent stability against reduction by li metal , 
 in particular , are exceptionally well suited for use as a protective 
 layer to enable li - metal based battery concepts .
llzo garnets 
 crystallize in a highly conductive cubic modification 
 ( sg : ia3d , no .
230 ) and a less conductive tetragonal polymorph ( space 
 group ( sg ) : i41/acd , 
 no .
the former is stabilized at 
 room temperature ( rt ) by supervalent substitution at the li , la , or 
 zr position in llzo .
the most promising , and extensively 
 studied , supervalent cations are al and ga , generally substituted 
 on the li sites .
much experimental and theoretical effort has been 
 expended to elucidate the site preferences of al and ga and their 
 influence on li - ion dynamics / conduction in llzo garnets .
additionally , it has been shown that the li - ion 
 conductivity of llzo stabilized with ga is twice that compared to 
 llzo stabilized with al . in order to understand this behavior better ,
cubic llzo was synthesized 
 by simultaneous substitution of al and ga in different ratios . in the corresponding li nmr line 
 shape measurements
an increase in li - ion dynamics with increasing 
 ga is observed , as yet the origin of this phenomenon remains , however , 
 unexplained .
a possible explanation was 
 found by some recent investigations on single crystals of li73xalxla3zr2o12 , with x = 0.10.4 and 
 li73ygayla3zr2o12 , with y = 0.10.6 by means of single crystal x - ray diffraction ( sc - xrd ) .
it was demonstrated that ga - stabilized llzo 
 crystallizes in the acentric  non - garnet  cubic space 
 group i43d , no .
220 , in contrast 
 to llzo ( see figure 1 for structural details ) .
( a ) crystal 
 structure of cubic llzo with space group ia3d ( no . 230 ) . blue dodecahedra ( 24c )
, tetrahedrally coordinated ( 24d ) sites represented by red spheres , octahedrally coordinated ( 48 g ) sites represented by yellow spheres , and distorted 4-fold 
 coordinated ( 96h ) sites represented by orange spheres . 

( c ) crystal 
 structure of cubic llzo with space group i43d ( no . 220 ) .
blue dodecahedra ( 24d ) are 
 occupied by la , green octahedra ( 16c ) by zr .
li are distributed over three sites , 
 two tetrahedrally coordinated sites 12a and 12b ( equivalent to 24d in ia3d ) ) represented by red and orange spheres , 
 respectively , and octahedrally coordinated ( 48e ) sites represented 
 by yellow spheres .
it was shown that the new space 
 group provides a different li - ion 
 diffusion mechanism leading to faster li - ion dynamics as shown by 
 nmr relaxometry experiments , recently .
the reasons for the phase transition and the relationship to the 
 macroscopic electrochemical properties , such as bulk ( bulk ) and grain boundary ( gb ) , li - ion conductivity , 
 activation energy ( ea ) , area specific 
 resistance ( asr ) , and microstructure , were , however , not fully understood . toward this end , in this work we combine powder x - ray powder diffraction 
 ( pxrd ) , sc - xrd , and neutron powder diffraction ( npd ) to characterize 
 samples through simultaneous refinement of the diffraction data .
this 
 combination of techniques helped us to obtain a detailed description 
 of the crystal structure .
scanning electron microscopy ( sem ) was used 
 to investigate the microstructure as a function of the al : ga ratio . 

impedance spectroscopy ( is ) measurements using both nonblocking electrodes 
 ( li ) and blocking electrodes ( ti|pt ) covering a wide temperature range 
 ( 120 to 40 c ) were used to characterize li - ion transport 
 and to study solid - state electrochemical properties .
finally , we used 
 density functional theory ( dft ) calculations to explain the decrease 
 in activation energy observed with ga substitution by carefully examining 
 changes in the energy landscape .
we show that , via lattice engineering 
 with ga substitution at optimized 
 crystallographic sites , the activation energy of ionic conduction 
 can be tailored for higher ionic conduction both in the bulk and at 
 the interfaces .
synthesis of li6.4al0.2xgaxla3zr2o12 garnets , with x = 0.00 , 0.05 , 
 0.10 , 0.15 , and 0.20 , was performed by a high - temperature sintering 
 route according ref ( 18 ) .
the starting materials were li2co3 ( 99% , 
 merck ) , la2o3 ( 99.99% , aldrich ) , zro2 ( 99.0% , aldrich ) , al2o3 ( 99.5% , aldrich ) , 
 and ga2o3 ( 99.0% , aldrich ) .
carbonates and oxides 
 in the stoichiometry of the desired composition with a 10% excess 
 of li2co3 were intimately ground together using 
 a hand mortar , a pestle , and isopropanol .
this mixture was pressed 
 uniaxially to form pellets , placed into a corundum crucible , and heated 
 to 850 c for 4 h with a heating rate of 5 c / min . to avoid 
 undesired contamination with al from the crucible
afterward the furnace was shut down 
 and the sample allowed to cool down naturally in the furnace to approximately 
 200 c . for the second and final step ,
the samples were milled 
 in isopropanol in a fritsch pulverisette 7 ball mill for 2 h ( 12 times 
 800 rpm for 5 min + 5 min break ) . finally , the powder was isostatically 
 pressed ( 24 kbar ) to form pellets and sintered at 1230 c for 
 6 h , with a heating rate of 20.5 c / min , and were allowed to 
 cool down to rt .
to avoid incorporation of al from the 
 crucible , the samples were again placed on a pellet of pure llzo . 
 to suppress formation of extra phases due to li loss during sintering
after 
 synthesis , samples were immediately packed under argon to avoid any 
 contact with moisture from the air ( for sc - xrd measurements only ) .
pxrd measurements were performed on powders from 
 the crushed pellets used for sc - xrd with a bruker d8 davinci design 
 diffractometer ( 280 mm goniometer radius , lynxeye solid state detector , 
 primary and secondary side soller slits , cu k radiation , collection 
 range 10120 2 ) . for lattice parameter refinements 
 using the program topas v2.1 ( bruker axs ) , phase
pure material was 
 mixed with silicon as an internal standard ( a = 5.43088 
  ) . for pxrd studies the remaining samples used for scxrd were 
 used .
sc - xrd data were collected on a bruker smart 
 apex ccd - diffractometer using mo k radiation .
small single 
 crystals up to 150 m were selected from the crushed pellets 
 after synthesis and sealed into glass capillaries to avoid prolonged 
 exposure to humidity .
intensity data were collected on samples within 
 48 h of their synthesis , using graphite - monochromatized mo k 
 x - radiation ( 50 kv , 30 ma ) .
the crystal - to - detector distance was 30 
 mm , and the detector was positioned at 30 and ( for some 
 points ) at 50 2 using an -scan mode strategy 
 at four different  positions ( 0 , 90 , 180 , 
 and 270 ) for each 2 position .
630 frames with  
 = 0.3 were acquired for each run . with this strategy
, data in 
 a large q - range up to minimum d - values d = 0.53  could be acquired .
three dimensional data were integrated 
 and corrected for lorentz , polarization , and background effects using 
 the apex2 software ( bruker , 2012 ) .
structure 
 solution ( using direct methods ) and subsequent weighted full - matrix 
 least - squares refinements on f were done 
 with shelx-2012 ( sheldrick , 2008 ) as implemented in the program suite 
 wingx 2014.1 ( farrugia , 2012 ) .
several crystallographic 
 positions show a mixed occupancy with li , al , and ga and vacancies . to overcome this ambiguity , 

special restrains were chosen : for the ga rich samples 
 with space group i43d , al was put onto the 12a position , while ga was allowed to distribute over 12a and 
 12b positions , together with li ; assuming 
 full occupancy of 12a and 12b sites 
 yields slightly to low ga contents , so vacancies were 
 introduced until the refined ga content met the one obtained 
 from edx analysis . a similar approach was used for samples with sg ia3d ; however , here the ga and al were directly fixed onto 24d positions while the li content was freely refined .
more 
 details on single crystal structure refinements can be obtained from 
 cifs with csd numbers : 430571 ( llzo : al0.20ga0.00 ) , 430574 ( llzo : al0.15ga0.05 ) , 430575 ( llzo : al0.10ga0.10 ) , 430576 ( llzo : al0.05ga0.15 ) , and 430603 ( llzo : al0.00ga0.15 ) .
the densities of pellets are calculated from the diameter , thickness , 
 and weight of the obtained pellets .
theoretical densities of pellets 
 are calculated from the cell parameters from the sc - xrd measurement .
the remaining samples used for sc - xrd were ground 
 and used for npd studies .
the neutron diffraction experiments were 
 done at the institut laue - langevin , ill , in grenoble ( france ) .
powder 
 diffraction data were acquired in constant wavelength mode (  
 = 1.5441  ) using the d20 diffractometer on 5 g batches 
 contained in 14 mm diameter vanadium sample cans at 298 k. experiments 
 were performed in the range 5.8  2  159.7 , 
 step width 0.04. an absorption correction was applied to the 
 neutron diffraction data .
hastings pseudo - voigt 
 function corrected for axial divergence , in conjunction with the d20 
 resolution function , was used to model peak shape .
after satisfactorily 
 refinement of neutron powder and sc - xrd data , the both data sets were 
 joined together , and simultaneous refinements were performed , the 
 results are discussed in text and tables . during mixed refinement , 
 the ga content ( when present ) was fixed to the value 
 obtained from edx analysis for all refinements , while the li and al content was allowed to freely refine for the ia3d structure . for sample llzo : al0.05ga0.15 ,
both the ga and al contents were fixed to the edx values , assuming that they 
 occupy the li1 site only as evidenced from sc - xrd data , while the 
 li content was allowed to adjust unconstrained . for sample 
 llzo : al0.00ga0.20
no stable refinements could be achieved putting al or 
 ga onto the interstitial li3 site .
in particular , we put emphasis on the investigation of the 
 grain size , morphology , and phase composition , and the al and ga content 
 using a backscattered electrons detector ( bse ) and energy - dispersive 
 x - ray spectroscopy ( edx ) measurements , respectively .
is ( impedance spectroscopy ) measurements were carried 
 out to investigate li - ion conductivities .
pt thin films were sputter 
 deposited with a thickness of 200 nm on top of ca .
10 nm ti ( used 
 to improve the adhesion between the sample and the electrode ) . for 
 the is measurements a novocontrol alpha analyzer was used in the frequency 
 range of 3  10 to 10 hz .
a julabo f-25 
 he circulator was used for cooling and partly also heating the samples 
 under investigation
. set temperatures between 12 and 25 c 
 ( partly 40 c ) were used , leading to true sample temperatures 
 from ca .
8 to 36 c . in the following , true sample temperatures , 
 measured by a thermocouple ,
an additional 
 impedance spectrum was recorded for a li / garnet / li sample at room 
 temperature in an ar glovebox . for this , metallic lithium was first 
 applied on the surfaces of the pellet , and the pellet was sandwiched 
 with two lithium foil disks in a swagelok type cell .
impedance data down to 120 c were recorded 
 with a novocontrol concept 80 spectrometer that is connected to a 
 quatro cryo system and equipped with a zgs active sample cell ( novocontrol ) .
a single ga or al ion 
 was placed onto the 24d site of the ia3d crystal structure with parameters taken 
 from sc - xrd measurements , and then an enumeration algorithm was used 
 to generate structures with one li placed into each of the distinct 
 remaining sites ( i.e. , 24d , and 96h ) .
ernzerhof ( pbe ) generalized - gradient 
 approximation ( gga ) , implemented in the vienna ab initio simulation 
 package ( vasp ) .
the projector augmented - wave 
 ( paw ) method is used for representation of core states .
an energy cutoff of 520 ev and a k - point density of at least 1000/(number of atoms in the unit cell ) 
 was used for all computations , with a background charge added to compensate 
 for the lack of li . during the relaxation the structures with the 
 li in an octahedral site always relaxed to the nearest tetrahedral 
 site in good agreement with previous calculations .
the total energy difference between structures with li 
 in the tetrahedral site closest to and farthest from the supervalent 
 cation was calculated ( see below ) .
for the sake of simplicity , samples with formula li6.4al0.2xgaxla3zr2o12 are denoted llzo : al0.20xgax .
first , the microstructure as a function of the al : ga ratio was 
 investigated .
back scattered electron ( bse)sem micrographs 
 of the polished pellets are shown in figure 2 . since bse is sensitive to the atomic number , 
 phases with different compositions can be easily distinguished .
no 
 composition other than llzo was observed , which is in agreement with 
 pxrd and npd data .
the increase of ga in llzo : al0.20xgax is correlated with 
 a denser studded microstructure with better connected grains and smaller 
 pores .
in contrast , the increase of al leads simultaneously to more 
 pronounced separation of grains and increased grain sizes ( up to 200300 
 m ) .
the relative theoretical density for all samples is , however , 
 almost the same and amounts to 85.0(3)% .
the al and ga content ( al : ga ) 
 of li6.4al0.2xgaxla3zr2o12 , with x = 0.00 , 0.05 , 0.10 , 0.15 , and 0.20 , measured 
 by edx is 0.19:0.00 , 0.14:0.05 , 0.12:0.08 , 0.07:0.14 , and 0.00:0.21 , 
 respectively ( see also table 1 )
. bse - sem image of polished embedded pellets of li6.4al0.2xgaxla3zr2o12 ; from left to right , x = 0.00 , 0.05 , 0.10 , 0.15 , and 0.20 .
lattice parameter a0 is given 
 in  ; site occupation values in atoms 
 per formula unit ( pfu ) .
polycrystalline samples of llzo : al0.20xgax with x = 0.000.20 
 were obtained from the pellets and used for the structure determination 
 ( xrd , sc - xrd , npd ) .
analysis of systematic extinctions of bragg peaks 
 in the single crystal data sets of the al - rich compositions unambiguously 
 yield the common garnet space group ia3d for llzo : al0.20ga0.00 , llzo : al0.15ga0.05 , and llzo : al0.10ga0.10 . for compositions llzo : al0.05ga0.15 and llzo : al0.00ga0.20 ,
the acentric space group i43d was observed as described in detail by 
 wagner et al .
the li - ion distribution as well as the lattice parameter 
 as a function of the proportion of ga is illustrated in figure 3 .
lattice parameter ( a0 ) ( a ) and li site 
 distribution ( b ) in li6.4al0.2xgaxla3zr2o12 , with x = 0.00 , 0.05 , 0.10 , 0.15 , 
 and 0.20 . on the basis of single crystal 
 structure refinements of samples 
 synthesized under the specific conditions
as set out in the experimental section it is assumed that in sg ia3d al and ga is enriched on the 
 tetrahedral 24d sites in llzo ; the 16a site is fully occupied by zr , and the 24c site contains la and a small amount of vacancies . with 
 increasing ga
content the amount of vacancies on 24c tends to decrease ; there is , however , no clear picture from xrd 
 data . in combined refinements ,
> 0.10 pfu 
 a 
 change in space group symmetry to i43d is observed .
for the latter sg there is strong evidence 
 that ga and al are enriched onto the tetrahedral 12a site ; this is observed in both single crystal x - ray diffraction 
 and data from combined refinement ( sc - xrd and npd ) and supported by 
 dft calculations .
the concentration of vacancies seems to be lower 
 on 12a and 12b sites in i43d sg as compared to ia3d .
in addition to the tetrahedral site(s ) , 
 li is also found on 96h and 48e positions , 
 respectively .
consequently , the overall content of 
 li is right above the ideal value of 6.40 pfu for 0.20 pfu 
 trivalent cations substituted .
however , considering the lower la content 
 ( according to simultaneous refinement of diffraction data ) , the li 
 content is in good agreement according charge neutrality .
moreover , 
 an increase of li occupation at the 24d site is observed . 

this behavior was suggested to be responsible for the decrease in 
 electrochemical performance . in the present 
 study ,
however , electrochemical properties seem to be improved by 
 the li occupation behavior observed ( see below ) .
the replacement 
 of al by ga slightly 
 increases the lattice parameter a0 , and 
 this finding is evident from the single crystal data .
the change of 
 symmetry , however , is not well pronounced in the variation of lattice 
 parameters within the compositions . in order to investigate 
 the influence of space group and microstructure 
 on b , gb , and ea , impedance spectra were measured using blocking electrodes 
 ( ti|pt ) for all compositions at temperatures between 120 and 
 40 c .
impedance spectra 
 of li6.4al0.2xgaxla3zr2o12 ( x = 0.00 , 0.05 , 0.10 , 0.15 , 
 and 0.20 ) samples at 20 c ( a ) , 80 c ( b ) , and dotted 
 fit / simulation lines ( equivalent circuit : rb  cpeb  rel  cpeel ) are included . 

data reflects resistivity  ( normalized to the sample area and 
 thickness ) . at 20 c ,
all samples 
 show a more or less complete high frequency 
 semicircle followed by a strong increase of the imaginary part of 
 the impedance toward low frequencies with an almost constant angle 
 in the complex impedance plane .
the semiarcs can be fitted by 
 a constant phase element ( cpeb ) in parallel to a resistance 
 element ( rb ) .
the capacitance cb calculated from the fit parameter qb and n ( c =   ( rq ) ) is in the pf range ( refined 
 from the equivalent circuit ) and the calculated relative permittivity 
 is about 40
. taken together these data suggest the high frequency 
 arc is attributed to a bulk process .
the 
 low frequency spike is well separated from the high ( or intermediate 
 arc at lower temperatures  see below ) and can be attributed 
 to the interface with electrodes . in a certain frequency range adding 
 another rel - cpeel element ( or 
 cpeel only at lower temperatures ) improves the fit .
this 
 electrode equivalent circuit helps with analysis of the sample - specific 
 high frequency features but does not imply any mechanistic information .
no indications of fast or resistive grain boundary contributions 
 are observed at ambient temperatures .
however , as the temperature 
 decreased below 20 c a slightly depressed semiarc at 
 intermediate frequencies was observed in the plots . in some cases 
 this intermediate arc
could be fitted by another serial rgb - cpegb element . due to the large uncertainty 
 in the capacitance cgb of the intermediate 
 arcs ( 10 to 10 f )
, the calculated 
 thicknesses can not be used to determine accurate normalized gb values according to the brick - layer model .
the capacitance obtained represents grain boundary processes , and 
 the corresponding activation energy can be calculated using the arrhenius 
 equation ( ea , gb = 0.32(4 ) ev ) .
the resulting 
 equivalent circuits are shown in the inset of figure 4 ; they fit all measurement data acceptably 
 well ( see dotted lines in figure 4 ) .
the temperature dependencies of bulk ( blocking 
 electrodes ) are shown in figure 5 for all samples .
the obtained bulk , ea , and asr values are given in table 2 . in order to determine 
 the asr as a function of the al :
ga ratio , identically prepared samples 
 of llzo : al0.20xgax sandwiched between li electrodes were used .
temperature dependent bulk conductivities for 
 li6.4al0.2xgaxla3zr2o12 ( x = 
 0.00 , 0.05 , 0.10 , 0.15 and 0.20 ) . at ambient temperatures
, bulk of 
 li ions in our llzo samples was determined from rb by using bulk = d / rba .
impedance spectra of li6.4al0.2xgaxla3zr2o12 ( x = 
 0.00 , 0.05 , 0.10 , 0.15 
 and 0.20 ) . for the sake of comparison ,
the data are normalized to 
 the geometry (  = rad , 
 with a = area and d = thickness ) . 

the solid lines represents the fit / simulation of llzo : al0.20xgax , with x = 0.00 , 0.05 , 0.10 , and 0.15 using the equivalent circuit shown 
 in the inset . for sample al0.00ga0.20
the nyquist plots of cells containing llzo : al0.20xgax , with x = 0.00 , 0.05 , 0.10 , and 0.15 are composed 
 of high frequency arcs , 
 clearly visible intermediate frequency arcs , and a low frequency feature . 
 as with the data obtained by fitting the spectra obtained on the cells 
 with blocking electrodes ,
cpeb element for the cells 
 containing samples with x = 0.000.15 . in 
 the case of llzo with
x = 0.20 , due to the invisible 
 high frequency arc , an r element only was used for 
 the fitting procedure .
since 
 any intermediate arc can represent grain boundaries or the interface , 
 we used the real capacitance calculated based on the cpeint value to distinguish between these contributions . since the cpeint values are about 0.1 f , typical of sample 
electrode 
 interface values , and no grain boundary contributions were observed 
 at rt in the blocking electrode experiments , we can assign the intermediate 
 arc to interface processes .
the very small low frequency feature corresponds 
 to an electrode response , which is typical for li electrodes and will 
 not be further considered in this study .
the resulting equivalent circuit used for all samples is displayed 
 in the inset of figure 6 .
as shown in figure 7a , the total values (= bulk above 
 20 c ) obtained by using blocking ( blue circle ) and ohmic 
 ( red squares ) electrodes are in very good agreement and increase almost 
 linearly as a function of the ga content ( slope 4.4  10 s cm/0.1 ga pfu ) .
the total values of llzo : al0.20ga0.00 are 
 very similar to values reported previously .
significantly , the total value of llzo : al0.00ga0.20 is one of the highest values found for 
 li - oxide garnets . comparably high values were only 
 reported by bernuy - lopez et al . as well as li et al . for li6.4ga0.2la3zr2o12 ( total = 1.0  10 s cm at 25 c ) and li6.4la3zr1.4ta0.6o12 , ( total = 1.3  
 10 s cm at 25 c ) , respectively 
 ( although the total values of samples studied herein 
 are measured at 20 c ) .
the values are very 
 close to the li - ion conduction limit suggested by jalem et al . on 
 the basis of force field based simulations ( bulk = 1.7  10 s cm ) .
activation 
 energy ( ea ) as a function 
 of the al : ga portion in li6.4al0.2xgaxla3zr2o12 ( x = 0.00 , 0.05 , 0.10 , 0.15 , 
 and 0.20 ) . a significant decrease in ea for x = 0.05 and 0.15
the gray areas at x = 0.00 and 0.20 indicate values obtained from experiment ( exp . ) 
 and calculations ( calc . ) from literature . in this study ,
they have resolved the different contributions 
 of li - ion conduction in bulk and grain boundaries in hot pressed llzo 
 solid electrolytes and found that bulk resistance dominates at temperatures 
 higher than 10 c .
they 
 also observed that the li - ion conductivity in llzo increases with 
 decreasing grain size and increasing concentration of grain boundaries . 

as noted above , for total ( bulk = total at ambient temperatures ) , 
 the increase 
 from llzo : al0.20ga0.00 to llzo : al0.00ga0.20 ( see figure 7a ) is almost linear , and no spontaneous increase in total , e.g. , caused by a phase transition , was observed .
on 
 the other hand , changes in activation energy of the samples in this 
 study ( see figure 7b ) show two distinct drops .
llzo : al0.20ga0.00 is characterized by an activation energy of 0.31 ev that is similar 
 to values reported previously ( about 0.260.37 ev ) . with the incorporation of 0.05 
 ga pfu into the llzo structure ( sg : ia3d )
a significant decrease of ea of about 0.03 ev was observed . a second drop in ea of 0.02 ev was seen at x = 0.15 .
llzo : al0.00ga0.20 is about 0.26 ev , which is lower than 
 previously reported ( 0.300.37 ev ) but similar to computed values ( 0.240.30 
 ev ) . to understand the first drop 
 in activation energy we calculated 
 site energy differences using dft .
the migration pathway for li - ion 
 motion involves a series of transitions between tetrahedral and neighboring 
 octahedral sites .
the low energy sites are tetrahedral , but as the 
 li - ion concentration increases , the li ions occupy the higher energy 
 octahedral sites . in order for the li 
 ion to migrate throughout the crystal structure
, they must pass through 
 the tetrahedral site located close to the supervalent cation .
we performed dft calculations on structures 
 with a single al or ga cation and a single 
 li with a compensating background charge and then computed 
 the total energy difference in structures with the li ion close to , 
 or far from , the cation as shown in figure 8a . ( a ) ia3d structures used 
 for site energy difference calculations .
( b ) subtracting 
 the difference of the total energy calculations for the nearest less 
 the farthest configurations .
it is evident that al is 
 less repulsive than ga , and thus the ga acts 
 to smooth the energy landscape more than al .
our results indicate that ga raises 
 the site energy 
 of the neighboring tetrahedral site by 10 mev more than al ( figure 8b ) .
in essence 
 this increase in site energy acts to smooth the energy landscape by 
 decreasing the site energy difference between the tetrahedral and 
 octahedral sites .
the rest of the improvement is likely associated 
 with the increase in grain size and grains connectivity .
the 
 second decrease in activation energy coincides with a phase 
 change to sg i43d at x = 0.15 , similar to what was seen in recent nmr spectroscopy 
 results of ga stabilized llzo with sg i43d . in that study ,
an additional 
 diffusion - induced relaxation rate peak in spin - lock li 
 nmr experiments at low temperatures indicated a further diffusion 
 process for llzo stabilized with ga ( x = 0.20 , sg : i43d ) , in contrast to samples stabilized 
 with al ( x = 0.00 , ia3d ) .
ab initio molecular dynamics support these findings 
 showing more facile diffusion in the i43d structure . furthermore
, the ea values of ga stabilized llzo were slightly lower compared to al 
 stabilized llzo . the area - specific 
 resistance ( asr ) of llzo : al0.20ga0.00-containing 
 cells turned out to be 77.8  cm , which 
 is similar to values reported previously ( see figure 7c ) .
the lowest 
 asr value of 37  cm was recently obtained 
 by cheng et al . for cells containing samples 
 with similar composition .
they found a strong 
 correlation between the asr and the microstructure of the llzo solid 
 electrolyte ; in particular , the asr was lower for samples in which 
 the surfaces of the llzo has a finer - grained microstructure and more 
 grain boundaries .
on the basis of this circumstance the higher asr 
 value obtained for llzo : al0.20ga0.00 herein 
 might be attributed to the larger average grain size of the sample . 

the trend observed in this study , in which asr decreased for samples 
 as ga content rose , may be in part due to changes in the microstructure . 

there was a significant decrease in asr of about 50  cm to values in the range of 24 to 28  cm for the samples containing ga , the lowest reported 
 values for llzo solid electrolytes , as far as we know .
in summary , a phase transition from ia3d to i43d occurs 
 with a critical amount of 0.15 ga pfu in ga and al cosubstituted samples 
 with general composition li6.4al0.2xgaxla3zr2o12 ( 0  x  0.2 ) . 

the increase in ga does not change the lattice parameter and the site 
 distribution of substituent cations significantly but leads to a preference 
 of the li ions to occupy the 24d sites ( or the equivalent 
 sites in the i43d structure ) .
the change in 
 structure coincides with an increase in the bulk li - ion conductivity 
 from 3.0  10 s cm for 
 0 ga pfu to 10 s cm for 0.20 
 ga pfu , with two significant drops in the activation energy at x = 0.05 and 0.15 .
dft calculations show that the first 
 drop in activation energy is largely related to ga 
li repulsion , 
 which acts to smooth the li - ion diffusion energy landscape compared 
 to al ; the second drop is due to the phase transition from ia3d to i43d .
this , combined with the changes in microstructure , seems 
 to be the explanation for the almost linear increase in li - ion conduction . 

an additional beneficial effect of the ga substitution is a decrease 
 in the interfacial resistance to values that are , to our knowledge , 
 the lowest ever reported values of llzo samples .
the success 
 in making a dense llzo sample with a li - ion conductivity 
 above 10 s cm and an asr of 
 about 20  cm described in this work bodes 
 well for the fabrication of devices with lithium anodes and llzo solid 
 electrolytes .
the present study showed how important an in - depth understanding 
 of the structure  property relationships in this class of materials 
 is if we want to advance in developing new electrochemical energy 
 storage devices .
 several  beyond li - ion battery 
  
 concepts such as 
 all solid - state batteries and hybrid liquid / solid systems envision 
 the use of a solid electrolyte to protect li - metal anodes . 
 these configurations 
 are very attractive due to the possibility of exceptionally high energy 
 densities and high ( dis)charge rates , but they are far from being 
 realized practically due to a number of issues including high interfacial 
 resistance and difficulties associated with fabrication . 
 one of the 
 most promising solid electrolyte systems for these applications is 
 al or ga stabilized li7la3zr2o12 ( llzo ) based on high ionic conductivities and apparent stability 
 against reduction by li metal . 
 nevertheless , the fabrication of dense 
 llzo membranes with high ionic conductivity and low interfacial resistances 
 remains challenging ; it definitely requires a better understanding 
 of the structural and electrochemical properties . in this study , the 
 phase transition from garnet ( ia3d , no . 
 230 ) to  non - garnet  
 ( i43d , no . 
 220 ) space group as a function of composition and 
 the different sintering behavior of ga and al stabilized llzo are 
 identified as important factors in determining the electrochemical 
 properties . 
 the phase transition was located at an al : ga substitution 
 ratio of 0.05:0.15 and is accompanied by a significant lowering of 
 the activation energy for li - ion transport to 0.26 ev . 
 the phase transition 
 combined with microstructural changes concomitant with an increase 
 of the ga / al ratio continuously improves the li - ion conductivity from 
 2.6  104 s cm1 to 1.2 
  103 s cm1 , which is close 
 to the calculated maximum for garnet - type materials . 
 the increase 
 in ga content is also associated with better densification and smaller 
 grains and is accompanied by a change in the area specific resistance 
 ( asr ) from 78 to 24  cm2 , the lowest reported value 
 for llzo so far . 
 these results illustrate that understanding the structure  
 properties 
 relationships in this class of materials allows practical obstacles 
 to its utilization to be readily overcome . 